When creating a DiffCommit with committer_date=None, which could

happen for some commits, we'd see a confusing error:
TypeError: 'committer_date' is an invalid keyword argument for this
function

This was a bit confusing because, although committer_date is a

property and not a model field, it clearly works in most situations

(we've been creating multi-commit review requests for some time now).
The failure only manifests when passing None as the value, which led

to a crash in the property when trying to call a method on it. This

raised an AttributeError, which triggered a handler in Django's

BaseModel class, making it think that committer_date didn't exist.
This change fixes the property to work with None values. It also

applies the same logic to author_date, so that if a None is passed

in, we'll just set None in the fields, allowing Django's own

validation to occur.
Unit tests were added for both of these properties.

Unit tests pass.
Hit this in production use and verified the fix.