Fix response_model being set to None for non-generator endpoints

fastapi/routing.py

@@ -1022,10 +1022,11 @@ def _populate_api_route_state(
             response_model = None
         else:
             stream_item = get_stream_item_type(return_annotation)
-            if stream_item is not None:
-                # Extract item type for JSONL or SSE streaming when
-                # response_class is DefaultPlaceholder (JSONL) or
-                # EventSourceResponse (SSE).
+            if stream_item is not None and is_generator:
+                # Extract item type for JSONL or SSE streaming for
+                # generator endpoints when response_class is
+                # DefaultPlaceholder (JSONL) or EventSourceResponse
+                # (SSE).
                 # ServerSentEvent is excluded: it's a transport
                 # wrapper, not a data model, so it shouldn't feed
                 # into validation or OpenAPI schema generation.