MonitorField - Change default to None when the field is nullable

[gmcrocetti]

Problem

MonitorField's default value is django.utils.timezone.now even when null=True. This behavior is misleading and should be changed. Despite that, some users might rely on this very same behavior and that's why #577 introduced a deprecation warning when such behavior is encountered.

The goal of this PR is make the field use None as default when initialized with null=True and without a default - of course the provided argument for default takes precedence.

Original discussion in #100

Solution

Remove the deprecation warning introduced in #577 and implement the desired behavior: use None when the field is nullable and no default is provided.

This change requires a major release (5.x.x) as discussed with @foarsitter in #577 .

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).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.94%. Comparing base (714632e) to head (952cbe2).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #599      +/-   ##
==========================================
- Coverage   98.94%   98.94%   -0.01%     
==========================================
  Files           6        6              
  Lines         760      757       -3     
==========================================
- Hits          752      749       -3     
  Misses          8        8              

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[foarsitter]

@gmcrocetti can you rebase this PR? Then I will merge it.

[gmcrocetti]

Just did it @foarsitter . Thanks ! :)

[foarsitter]

Thanks @gmcrocetti