fix: fix ARQ integration error (#4427)

[ninoseki]

Fix #4427 by setting an empty list if settings_cls.cron_jobs is None.

[sentrivana]

Thanks for reporting and fixing @ninoseki!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.60%. Comparing base (aad481d) to head (99650dd).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4428      +/-   ##
==========================================
- Coverage   80.61%   80.60%   -0.01%     
==========================================
  Files         142      142              
  Lines       15982    15982              
  Branches     2729     2729              
==========================================
- Hits        12884    12883       -1     
  Misses       2241     2241              
- Partials      857      858       +1     

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

... and 1 file with indirect coverage changes