Update fillAttribute to return parent result by default

[mikeaag]

After upgrading to v2.0.0 I was getting the following error when trying to save a resource that included a CKEditor field

Waynestate\Nova\CKEditor4Field\CKEditor::fillAttribute(): Return value must be of type ?callable, none returned

It looks like this was caused by the addition of the return type. If there is no pending upload, then the method does not return, which is apparently a type error.

Being completely honest, I am not sure on the ramifications of my suggested change. I initially changed it to just return null, which fixed the error for me. Then I changed it to return the result of parent::fillAttribute(), which also works for me.

Where my uncertainty comes in is when there are uploads to handle. We do not use the upload functionality so I can't easily test to make sure that still works as expected. Would be great if someone who does have it setup to do uploads could give this a test to make sure it works for them still.

[chrispelzer]

I have a vanilla nova set up that I can test it on. I'll double check all the draft/uploads/pruning.