Fix case when cron_jobs is None in Arq settings class

Arq settings can be defined as a dictionary or a class.

When using a class inheriting from `WorkerSettingsBase`,
the default value for `cron_jobs` is `None`.

This causes a `TypeError: 'NoneType' object is not iterable`
exception in Sentry when starting the worker.

This fix is a followup to getsentry#4115
which handled the issue for the other settings formats.

Author: ronnix

sentry_sdk/integrations/arq.py

@@ -210,11 +210,13 @@ def _sentry_create_worker(*args, **kwargs):
 
         if hasattr(settings_cls, "functions"):
             settings_cls.functions = [
-                _get_arq_function(func) for func in settings_cls.functions
+                _get_arq_function(func)
+                for func in getattr(settings_cls, "functions", [])
             ]
         if hasattr(settings_cls, "cron_jobs"):
             settings_cls.cron_jobs = [
-                _get_arq_cron_job(cron_job) for cron_job in settings_cls.cron_jobs
+                _get_arq_cron_job(cron_job)
+                for cron_job in getattr(settings_cls, "cron_jobs", [])
             ]
 
         if "functions" in kwargs: