fix console warning about unsupported data format

before this change you could see the following warning and error on the
browser console:

Can not convert Object {  }
runRequest.catchError Error: Unsupported data format

when changing from "predefined graph" to "single metric" with an already
selected predefined graph.

Author: BenediktSeidl

src/backend/rest.ts

@@ -362,7 +362,10 @@ export default class RestApiBackend implements Backend {
       // this will result in a grafana note that suggests the graph should be
       // changed to a table. By returning an empty MutableDataFrame grafana
       // shows "no data" as expected.
-      return toDataFrame({});
+      return toDataFrame({
+        refId: query.refId,
+        fields: [],
+      });
     }
   }