fields_for_model returns all fields when fields parameter is the empty tuple

[alexdutton]

When attempting to use modelformset_factory to generate forms where the user can only delete objects (i.e. not edit them), one would expect a fields=() to create forms with only delete checkboxes. However, fields_for_model uses the following check to determine whether to restrict fields:

 if fields and not f.name in fields:
     continue

When passed the empty tuple, fields are never skipped as bool( () ) == False, resulting in all fields being displayed.

This should be:

 if fields is not None and not f.name in fields:
     continue

I've attached a patch for both construct_instance (which also has the wrong behaviour) and fields_for_model.

The additional regression tests cover construct_instance, fields_for_model, modelformset_factory and ModelForm. The latter is included even though the usage of fields=() in the this case is arguably nonsensical.

[alexdutton]

Patch and regression tests

[Honza Král]

(In [14199]) Fixed #14119 -- fields_for_model no longer returns all fields when fields parameter is the empty tuple. Thanks alexdutton!