Fix AttributeError when optional settings are undefined

Fixes #1328
- Add None as default value in axes_conf_check
- Add test coverage for missing settings scenario

axes/checks.py

@@ -207,7 +207,7 @@ def axes_conf_check(app_configs, **kwargs):  # pylint: disable=unused-argument
     ]
 
     for callable_setting in callable_settings:
-        value = getattr(settings, callable_setting)
+        value = getattr(settings, callable_setting, None)
         if not is_valid_callable(value):
             warnings.append(
                 Warning(

tests/test_checks.py

@@ -129,3 +129,8 @@ class ConfCheckTestCase(AxesTestCase):
     def test_valid_callable(self):
         warnings = run_checks()
         self.assertEqual(warnings, [])
+
+    def test_missing_settings_no_error(self):
+        warnings = run_checks()
+        self.assertEqual(warnings, [])
+