`BaseFormSet`'s `media` and `is_multipart` don't work correctly when `extra=0`

[Simon Charette]

When generating a BaseFormSet subclass with no extra forms ​the underlying form's medias are ignored.

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In [1]: from django import forms

In [2]: class MediaForm(forms.Form):
   ...:     class Media:
   ...:         js = ('some-file.js',)
   ...:         

In [3]: str(forms.formsets.formset_factory(MediaForm)().media)
Out[3]: '<script type="text/javascript" src="/static/some-file.js"></script>'

In [4]: str(forms.formsets.formset_factory(MediaForm, extra=0)().media)
Out[4]: ''

[Simon Charette]

I'm wondering if we should also decorate BaseFormSet.empty_form with cached_property in order to avoid generating a form instance just for accessing it's media property. This instance could be reused when rendering the formset.

[Claude Paroz]

I'm not so enthousiast to set a cached_property. This is not performance-critical code, and with caching, you always create invalidation issue (e.g. call empty_form before changing self.auto_id).
BTW, I noticed that empty_form had unusable kwargs parameters. I will fix that separately.

[Simon Charette]

Just noticed is_multipart has ​a similar issue.

Should I open another ticket for that or just fix it here and add another test?

[Claude Paroz]

Let's fix both issues in this ticket.

[Simon Charette]

Added a second patch for the is_multipart issue and changed ticket summary.

[Claude Paroz <claude@…>]

In 3fc43c964ef7ab52261ec3a164d66b19f06f5cea:

Fixed #19545 -- Make sure media/is_multipart work with empty formsets

[Claude Paroz <claude@…>]

In 70cc95d1ccab823d30d123d17e864c4867ee244b:

[1.5.x] Fixed #19545 -- Make sure media/is_multipart work with empty formsets

Backport of 3fc43c964e from master.