Avoid exception in changes_display_dict when model is missing

[ppmathis]

As django-auditlog v3 now stores the actual PKs for foreign models instead of their string representation, I switched my application from changes_dict to changes_display_dict so that the actual string-representation of foreign models is being shown again. While this worked in my local environment, it broke while trying out this change against a previously used testing database, specifically by raising a KeyError.

As it turns out, if a model has been previously registered with django-auditlog and made any entries, followed by then being no longer registered (e.g. if the model has been deprecated or further logging is no longer desired), any calls to changes_display_dict will fail. The reason behind this is that the method attempts to lookup the model within the registry, without checking if it actually still exists.

This lookup is only needed for respecting mapping_fields and has no other purpose. This PR extends the logic to first check if the model is contained in the registry, and only if that is the case, the actual lookup happens. If a previously existing model is being looked up instead, it will still show up properly, just without the mapped fields as they no longer exist.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d78f813) 95.20% compared to head (fa0fbba) 95.21%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #609      +/-   ##
==========================================
+ Coverage   95.20%   95.21%   +0.01%     
==========================================
  Files          31       31              
  Lines        1021     1025       +4     
==========================================
+ Hits          972      976       +4     
  Misses         49       49              

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[hramezani]

Thanks @ppmathis