extra being passed to SQLalchemy (#40391)

The edit prefills an empty extra field (None)
with a empty dictionary, which is decoded
to an extra field being passed in the con url.

(cherry picked from commit a856ed4)

airflow/www/views.py

@@ -4219,7 +4219,10 @@ def prefill_form(self, form, pk):
             if is_sensitive and field_name in extra_dictionary:
                 extra_dictionary[field_name] = SENSITIVE_FIELD_PLACEHOLDER
         # form.data is a property that builds the dictionary from fields so we have to modify the fields
-        form.extra.data = json.dumps(extra_dictionary)
+        if extra_dictionary:
+            form.extra.data = json.dumps(extra_dictionary)
+        else:
+            form.extra.data = None
 
 
 class PluginView(AirflowBaseView):