Fix a bug in config. None was getting converted to "None"

This broke out available_models API which checked for None.

libs/core/kiln_ai/utils/config.py

@@ -97,7 +97,8 @@ def __getattr__(self, name: str) -> Any:
 
         # Check if the value is in settings
         if name in self._settings:
-            return property_config.type(self._settings[name])
+            value = self._settings[name]
+            return value if value is None else property_config.type(value)
 
         # Check environment variable
         if property_config.env_var and property_config.env_var in os.environ:
@@ -110,7 +111,7 @@ def __getattr__(self, name: str) -> Any:
         else:
             value = property_config.default
 
-        return property_config.type(value)
+        return None if value is None else property_config.type(value)
 
     def __setattr__(self, name, value):
         if name in ("_properties", "_settings"):

libs/core/kiln_ai/utils/test_config.py

@@ -26,6 +26,7 @@ def config_with_yaml(mock_yaml_file):
                     str, default="default_value", env_var="EXAMPLE_PROPERTY"
                 ),
                 "int_property": ConfigProperty(int, default=0),
+                "empty_property": ConfigProperty(str),
             }
         )
 
@@ -67,6 +68,12 @@ def test_nonexistent_property(config_with_yaml):
         config.nonexistent_property
 
 
+def test_nonexistent_property_get_value(config_with_yaml):
+    config = config_with_yaml
+    assert config.get_value("nonexistent_property") is None
+    assert config.get_value("empty_property") is None
+
+
 def test_property_type_conversion(config_with_yaml):
     config = config_with_yaml
     config = Config(properties={"int_property": ConfigProperty(int, default="42")})