Why the data is still updating even the textbox is empty? (Vue 2/Axios/PHP)

Good day everyone! When I enter the customer_address it updates the database even if I have not yet input the customer_first_name, customer_last_name and customer_contact_no. But if I don’t input the address he won’t update the row in the database. What do you think is the problem? How can I prevent the input from being empty.

//customer.js
identifyCustomerId : function(customer_id) {
            for(var index = 0; index < this.customers.length; index++) {
                if(customer_id == this.customers[index].customer_id) {
                    this.customer_first_name = this.customers[index].first_name;
                    this.customer_last_name = this.customers[index].last_name;
                    this.customer_contact_no = this.customers[index].contact_no;
                    this.customer_address = this.customers[index].address;
                }
            }
        },
openToggleUpdateForm : function(customer_id) {
            this.updateForm = true;
            this.toggleButton = false;
            this.customer_id = customer_id;
            this.identifyCustomerId(customer_id);

        },
 userValidation : function() {
            if(this.customer_first_name.trim() === "") {
                this.first_name_error = true;
                this.first_name_error_message = "This field is required!";
                console.log(this.first_name_error);
            }
            else {
                this.first_name_error = false;
                this.first_name_error_message = "";
            }
            if(this.customer_last_name.trim() == "") {
                this.last_name_error = true;
                this.last_name_error_message = "This field is required!";
            }
            else {
                this.last_name_error = false;
                this.last_name_error_message = "";
            }
            if(this.customer_contact_no.trim() == "") {
                this.contact_no_error = true;
                this.contact_no_error_message = "This field is required!";
            }
            else {
                this.contact_no_error = false;
                this.contact_no_error_message = "";
            }
            if(this.customer_address.trim() == "") {
                this.address_error = true;
                this.address_error_message = "This field is required!";
            }
            else {
                this.address_error = false;
                this.address_error_message = "";
            }
        },
updateCustomer : function() {
            this.userValidation();
            console.log(this.customer_first_name);
            axios.post(this.urlRoot + this.api + "update_customer.php", {
                customer_id : this.customer_id,
                first_name : this.customer_first_name,
                last_name : this.customer_last_name,
                contact_no : this.customer_contact_no,
                address : this.customer_address
            }).then(function (response) {
                if(response.data.status == "NOT OK") {
                    vm.first_name_error = response.data.first_name_error;
                    vm.last_name_error = response.data.last_name_error;
                    vm.contact_no_error = response.data.contact_no_error;
                    vm.address_error = response.data.address_error;
                } else{
                    console.log(response);
                    vm.retrieveCustomer();
                }
            }); 
        }

//update_customer.php (api)
$customer = new Customer();

    $data = json_decode(file_get_contents("php://input"),true);

    $response = [];

    $response['status'] = "OK";

    if(empty($data['customer_id'])) {
        $response['customer_id_error'] = true;
        $response['status'] = "NOT OK";
    }
    if(empty($data['first_name'])) {
        $response['first_name_error'] = true;
        $response['status'] = "NOT OK";
        echo $data['first_name'];

    }
    if(empty($data['last_name'])) {
        $response['last_name_error'] = true;
        $response['status'] = "NOT OK";
    }
    if(empty($data["contact_no"])) {
        $response["contact_no_error"] = true;
        $response["status"] = "NOT OK";
    }
    if(empty($data['address'])) {
        $response['address_error'] = true;
        $response['status'] = "NOT OK";
    }
    else{
        $customer = new Customer();

        $response['status'] = "OK";

        echo json_encode($response);
        $customer->customer_id = ucwords($data['customer_id']);
        $customer->first_name = ucwords($data['first_name']);
        $customer->last_name = ucwords($data['last_name']);
        $customer->contact_no = $data['contact_no'];
        $customer->address = ucwords($data['address']);
        $customer->updateCustomer();
    }

//customer_page.php ("update button on my table")
<button class="btn btn-secondary" @click="openToggleUpdateForm(customer.customer_id)">Update</button>

//customer_page.php ("update form")
<div class="add-content" v-if="updateForm">
                <h1>Add Customer</h1>
                <label>Firstname: </label>
                <input type="text" name="first_name" :class="{ invalid : first_name_error }" v-model="customer_first_name" required><br>
                <div class="text-danger">{{ first_name_error_message }}</div>
                <label>Lastname: </label>
                <input type="text" name="last_name" :class="{ invalid : last_name_error }" v-model="customer_last_name" required><br>
                <div class="text-danger">{{ last_name_error_message }}</div>
                <label>Contact no.: </label>
                <input type="text" name="contact_no" :class="{ invalid : contact_no_error }" v-model="customer_contact_no" required><br>
                <div class="text-danger">{{ contact_no_error_message }}</div>
                <label>Address: </label>
                <input type="text" name="address" :class="{ invalid : address_error }" v-model="customer_address" required><br>
                <div class="text-danger">{{ address_error_message }}</div>
                <div class="mt-4"> 
                    <button @click="updateCustomer()" class="btn btn-primary">Add</button>
                </div>
            </div>