Coerce None values into strings in logentry params.

[antonpirker]

Nice rendering of log messages containing parameters that are None values does not work. There we coerce None values into strings to have nicer messages in Sentry UI.

Fixes #3660

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.56%. Comparing base (42ad8df) to head (909d94c).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4121      +/-   ##
==========================================
+ Coverage   79.49%   79.56%   +0.07%     
==========================================
  Files         140      140              
  Lines       15624    15624              
  Branches     2650     2651       +1     
==========================================
+ Hits        12420    12431      +11     
+ Misses       2369     2360       -9     
+ Partials      835      833       -2     

Files with missing lines	Coverage Δ
sentry_sdk/integrations/logging.py	89.10% <ø> (ø)

... and 5 files with indirect coverage changes

[sentrivana]

Nice.

Some tests also need updating, it seems. It'd also be great to have a test case for the None case specifically.