also make None a valid option for options API for #11854

modules/api/models.py

 import inspect
+import types
+
 from pydantic import BaseModel, Field, create_model
 from typing import Any, Optional
 from typing_extensions import Literal
@@ -207,11 +209,14 @@ class PreprocessResponse(BaseModel):
 fields = {}
 for key, metadata in opts.data_labels.items():
     value = opts.data.get(key)
+    if key == 'sd_model_checkpoint':
+        value = None
     optType = opts.typemap.get(type(metadata.default), type(value))
 
-    if (metadata is not None):
-        fields.update({key: (Optional[optType], Field(
-            default=metadata.default ,description=metadata.label))})
+    if optType == types.NoneType:
+        pass
+    elif metadata is not None:
+        fields.update({key: (Optional[optType], Field(default=metadata.default, description=metadata.label))})
     else:
         fields.update({key: (Optional[optType], Field())})