Handling of Apps without Migrations

[psyonara]

When you have apps in your Django project that do not have migrations (i.e. apps without any data models), running the migrations causes this error:

Traceback (most recent call last):
  File "manage.py", line 15, in <module>
    execute_from_command_line(sys.argv)
  File "/usr/local/lib/python3.7/site-packages/django/core/management/__init__.py", line 381, in execute_from_command_line
    utility.execute()
  File "/usr/local/lib/python3.7/site-packages/django/core/management/__init__.py", line 375, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/usr/local/lib/python3.7/site-packages/django/core/management/commands/test.py", line 23, in run_from_argv
    super().run_from_argv(argv)
  File "/usr/local/lib/python3.7/site-packages/django/core/management/base.py", line 323, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/usr/local/lib/python3.7/site-packages/django/core/management/base.py", line 364, in execute
    output = self.handle(*args, **options)
  File "/usr/local/lib/python3.7/site-packages/django/core/management/commands/test.py", line 53, in handle
    failures = test_runner.run_tests(test_labels)
  File "/usr/local/lib/python3.7/site-packages/django/test/runner.py", line 632, in run_tests
    self.run_checks()
  File "/usr/local/lib/python3.7/site-packages/django/test/runner.py", line 570, in run_checks
    call_command('check', verbosity=self.verbosity)
  File "/usr/local/lib/python3.7/site-packages/django/core/management/__init__.py", line 148, in call_command
    return command.execute(*args, **defaults)
  File "/usr/local/lib/python3.7/site-packages/django/core/management/base.py", line 364, in execute
    output = self.handle(*args, **options)
  File "/usr/local/lib/python3.7/site-packages/django/core/management/commands/check.py", line 64, in handle
    fail_level=getattr(checks, options['fail_level']),
  File "/usr/local/lib/python3.7/site-packages/django/core/management/base.py", line 390, in check
    include_deployment_checks=include_deployment_checks,
  File "/usr/local/lib/python3.7/site-packages/django/core/management/base.py", line 377, in _run_checks
    return checks.run_checks(**kwargs)
  File "/usr/local/lib/python3.7/site-packages/django/core/checks/registry.py", line 72, in run_checks
    new_errors = check(app_configs=app_configs)
  File "/usr/local/lib/python3.7/site-packages/django_linear_migrations/apps.py", line 81, in check_max_migration_files
    migration_details = MigrationDetails(app_label)
  File "/usr/local/lib/python3.7/site-packages/django_linear_migrations/apps.py", line 43, in __init__
    self.migrations_module = import_module(self.migrations_module_name)
  File "/usr/local/lib/python3.7/importlib/__init__.py", line 118, in import_module
    if name.startswith('.'):
AttributeError: 'NoneType' object has no attribute 'startswith'

From looking at the DLM source code, it seems that a None value for the migrations module isn't handled correctly.
In apps.py:43 it tries to import the migrations module even if that app doesn't have one.

[adamchainz]

Fixed in #24, released in 1.2.1.