Complex string manipulation by JavaScript regexp

I am generating some meaningful name with the following rule in a JavaScript/Node JS program:

Input: “tenancy_account__accountPublicId__workspace__workspacePublicId__remove-user__userPublicId“

Expected output: “TenancyAccountAccountPublicIdWorkspaceWorkspacePublicIdRemove-userUserPublicId”

Rules:

1. replace any character with zero or more underscore to the non-underscored uppercase
   Example:x | __*x => X
2. If exists remove last _

This is what is tried so far, looking for better alternatives, if any:

const convertBetterString = (input) => {
    const finalString = [];
    if (input && input.includes('_')) {
        const inputStringSeparation = input.split('_');
        if (inputStringSeparation.length > 1) {
            if (inputStringSeparation[inputStringSeparation.length - 1] === '') {
                inputStringSeparation.splice(inputStringSeparation.length - 1, 1);
            }
            inputStringSeparation.forEach((val, index) => {
                if (val === '' && inputStringSeparation[index + 1]) {
                    const actualString = inputStringSeparation[index + 1];
                    const formattedString = actualString.charAt(0).toUpperCase() + actualString.slice(1);
                    finalString.push(formattedString);
                }
            });
            return finalString.length > 0 ? finalString.join('') : inputStringSeparation.join('');
        } else {
            return input.charAt(0).toUpperCase() + input.slice(1);
        }
    } else {
        return input;
    }
}