How can I add a IF EMPTY to PHP Snippet?

This snippet works very fine, catch de url parameter and show it in a shortcode, but I want to put a conditional if this parameter is empty: IF “NOMBRE” IS EMPTY, SHOW FOR EXAMPLE THE TEXT “¡HOLA MUNDO!”.

function custom_shortcode() {
        return '' . htmlspecialchars($_GET["nombre"]) . ''; 
} 
add_shortcode( 'nombre', 'custom_shortcode' );