profile_has_required_custom_fields_set fails to detect if a custom profile field is empty

The profile_has_required_custom_fields_set introduced by MDL-46946 relies on a sql query to determine if there is some empty custom profile fields or not. Its base assumption is that a field is empty if and only if there is no value for it in {user_info_data}.

This assumption is not always true. Profile field plugins can override a function named is_empty() to instruct Moodle if a field is empty or not.

profile_has_required_custom_fields_set() should call is_empty() for each plugin.
It is possible that there is no value in {user_info_data} and yet the field is not empty. An example is the profilefield_associated plugin.