#7703 Note edge case compatibility break in 3.10.x

[Syntaf]

Short Summary: Adds a warning about a possible backwards compatibility break where fields having source='*' receiving values of None would now pass that None through the fields validation.

Tag-along PR to #7703 which improves 3.10.x release notes - explained in more detail here #7716 (comment)

[tomchristie]

I'm going to close this off, since I can't understand the wording in this change.
Perhaps there's something here that's worth starting a discussion on first?

[Syntaf]

Fair enough, I did mention in that comment that it's a rare edge case so no harm in closing this.

The basis of the compatibility break was that previous to 3.10.X, values of None passed into fields with source='*' skipped all validation.

This change however meant that from 3.10.X on, validation will always be ran on these fields regardless of value.

As a result, we had some CharField(source='*', allow_null=True) fields stop working after upgrading to 3.10.X because even with allow_null, None is passed through validation and thus fails where it previously didn't.