Fix update_change_reason: fails to fetch latest history entry for nullable fields

[Senor-Ducky]

Description

The update_change_reason method in utils.py fails to update change reason in case any field is nullable in a model or is set to default to None. This could be due to the way django handles null values, as direct comparison to None is not allowed when filtering.

Related Issue

#1475

Motivation and Context

Encountered this issue when dealing with a model with nullable fields in one of my projects

How Has This Been Tested?

Has been tested both within my project environment via test cases simulating this scenario, also tested within the suite by adding a new similar test case.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have run the pre-commit run command to format and lint.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have added my name and/or github handle to AUTHORS.rst
• I have added my change to CHANGES.rst
• All new and existing tests passed.

[ddabble]

Closing - see the issue discussion.