The problem lies in the explode_enum_string function. When the argument passed to the function is an empty string, it returns explode( ',', '' ). And that is an array with one empty value.

The solution is to return an empty array when an empty string is received. The function would be:

function explode_enum_string( $p_enum_string ) {
    if ( empty($p_enum_string) ) {
        return array();
    }
    return explode( ',', $p_enum_string );
}