Fix crash when mkdocs_theme.yml is empty

Fixes #3699

mkdocs/tests/theme_tests.py

@@ -104,3 +104,22 @@ class ThemeTests(unittest.TestCase):
                 ],
             )
             self.assertEqual(theme.static_templates, {'sitemap.xml', 'child.html', 'parent.html'})
+
+    def test_empty_config_file(self):
+        # Test for themes with *empty* mkdocs_theme.yml.
+        # See https://github.com/mkdocs/mkdocs/issues/3699
+        m = mock.Mock(
+            # yaml.load returns "None" for an empty file
+            side_effect=[None]
+        )
+        with mock.patch('yaml.load', m) as m:
+            theme = Theme(name='mkdocs')
+            # Should only have the default name and locale __vars set in
+            # Theme.__init__()
+            self.assertEqual(
+                dict(theme),
+                {
+                    'name': 'mkdocs',
+                    'locale': parse_locale('en'),
+                },
+            )

mkdocs/theme.py

@@ -138,6 +138,9 @@ class Theme(MutableMapping[str, Any]):
                 f"Please upgrade to a current version of the theme."
             )
 
+        if theme_config is None:
+            theme_config = {}
+
         log.debug(f"Loaded theme configuration for '{name}' from '{file_path}': {theme_config}")
 
         if parent_theme := theme_config.pop('extends', None):