fix: guard against null content_filters

[johnnagro]

20:38:43 2024-01-23 20:39:12,091 INFO 11552 [enterprise.management.commands.add_exec_ed_exclusion_to_catalogs] [user None] [ip None] add_exec_ed_exclusion_to_catalogs.py:62 - 3973a547-72c4-4cda-bd20-2fb66425a760
20:38:43 Traceback (most recent call last):
20:38:43   File "/edx/app/edxapp/edx-platform/manage.py", line 106, in <module>
20:38:43     execute_from_command_line([sys.argv[0]] + django_args)
20:38:43   File "/edx/app/edxapp/venvs/edxapp/lib/python3.8/site-packages/django/core/management/__init__.py", line 419, in execute_from_command_line
20:38:43     utility.execute()
20:38:43   File "/edx/app/edxapp/venvs/edxapp/lib/python3.8/site-packages/django/core/management/__init__.py", line 413, in execute
20:38:43     self.fetch_command(subcommand).run_from_argv(self.argv)
20:38:43   File "/edx/app/edxapp/venvs/edxapp/lib/python3.8/site-packages/django/core/management/base.py", line 354, in run_from_argv
20:38:43     self.execute(*args, **cmd_options)
20:38:43   File "/edx/app/edxapp/venvs/edxapp/lib/python3.8/site-packages/django/core/management/base.py", line 398, in execute
20:38:43     output = self.handle(*args, **options)
20:38:43   File "/edx/app/edxapp/venvs/edxapp/lib/python3.8/site-packages/enterprise/management/commands/add_exec_ed_exclusion_to_catalogs.py", line 64, in handle
20:38:43     if customer_catalog.content_filter.get('course_type__exclude'):
20:38:43 AttributeError: 'NoneType' object has no attribute 'get'non-zero return code