Fix refresh wiping all links when JSON file is missing

When the file at json_path didn't exist, clicking Refresh Outputs
returned empty keys causing all outputs and links to be removed.
Now the API returns an error flag and the frontend bails out early,
preserving existing outputs.

Bump version to 1.2.1.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

json_loader_dynamic.py

@@ -63,6 +63,8 @@ if PromptServer is not None:
             seq = 1
         data = read_json_data(json_path)
         target = get_batch_item(data, seq)
+        if not data:
+            return web.json_response({"keys": [], "types": [], "error": "file_not_found"})
         keys = []
         types = []
         if isinstance(target, dict):