Fix LogEntry.changes_dict() to return {} when json.loads() returns None

[tykling]

LogEntry.changes_str raises an exception when viewing the new "accessed" AuditLog objects with no changes. This PR fixes LogEntry.changes_dict() so it always returns a dict, even if json.loads() returns None.

This PR fixes #471 which has the backtrace and more details 👍

[hramezani]

Thanks @tykling for reporting the issue and preparing the fix.

Please consider to:

• Add a test for the fix
• Add changelog entry in https://github.com/jazzband/django-auditlog/blob/master/CHANGELOG.md

[codecov]

Codecov Report

Merging #472 (561228e) into master (caf5daa) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #472   +/-   ##
=======================================
  Coverage   92.74%   92.74%           
=======================================
  Files          26       26           
  Lines         786      786           
=======================================
  Hits          729      729           
  Misses         57       57           

Impacted Files	Coverage Δ
auditlog/models.py	87.65% <100.00%> (ø)

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[tykling]

done

[hramezani]

@tykling Thanks for the update. Please add a regression test. I mean you need to add a test that fails without your fix to show what problem you are fixing.

[tykling]

I did and it does

[hramezani]

Ah Sorry. You are right! Thanks @tykling 👍