Fixed bug when cron_jobs is set to None in arq integration

[antonpirker]

Handle None values in arq configuration gracefully.

Fixes #3827

Unfortunately our test setup for arq does not allow for testing None values in the configuration because they are changed to [] in the arq init code in the test suite here and here.

This is why I have not added additional tests for this bug fix.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.57%. Comparing base (0d23b72) to head (ba99d03).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4115      +/-   ##
==========================================
- Coverage   79.60%   79.57%   -0.04%     
==========================================
  Files         140      140              
  Lines       15581    15581              
  Branches     2643     2643              
==========================================
- Hits        12404    12399       -5     
- Misses       2338     2343       +5     
  Partials      839      839              

Files with missing lines	Coverage Δ
sentry_sdk/integrations/arq.py	89.84% <ø> (ø)

... and 5 files with indirect coverage changes

[szokeasaurusrex]

Maybe we could make a test, which does not use the init_arq test fixture, to test this? But if it is too complicated, let's not block the bug fix on a lack of a test.