Refs #6509
This enforces allow_empty=True when a ListSerializer is a child of another serializer and partial validation is being performed on the parent serializer.
This is because partial validation should allow fields to be omitted, but should not cause values that are invalid without partial validation to become valid.
This effectively reverts #4222. None of the tests added in that PR fail if the associated change is removed, so I‘m not sure what that PR was trying to fix.
Calling dict.keys() is unnecessary. The two are functionally equivalent
on modern Pythons.
Inspired by Lennart Regebro's talk "Prehistoric Patterns in Python" from
PyCon 2017.
https://www.youtube.com/watch?v=V5-JH23Vk0I
