feat(monitor): Change default to None when the field is nullable

[gmcrocetti]

Problem

I recently came across a non-intuitive behavior, IMHO. The MonitorField has its default value set to now even though marked as nullable. Please, I'm open to hearing the opinion case it's is expected behavior.

Solution

After catching up the discussion in #100 I decided to implement it. The discussed solution is to set its default value to None when null=True.

If might break backward compatibility the edge case of someone marking the field as null=True and not expecting default to be None.

Commandments

• Write PEP8 compliant code.
• Cover it with tests.
• Update CHANGES.rst file to describe the changes, and quote according issue with GH-<issue_number>.
• Pay attention to backward compatibility, or if it breaks it, explain why.
• Update documentation (if relevant).