Sanitize ovs br-get-external-id output on openvswitch_bridge (#24209)

If a bridge does not have external_ids, ovs-vsctl returns '{}'.
This causes issues on the current want vs have comparison in cases
where the play does not define external_ids, as the comparison
is None vs '{}'.

lib/ansible/modules/network/ovs/openvswitch_bridge.py

@@ -117,6 +117,9 @@ from ansible.module_utils.six import iteritems
 from ansible.module_utils.pycompat24 import get_exception
 
 def _external_ids_to_dict(text):
+    if not text:
+        return None
+    else:
         d = {}
 
         for l in text.splitlines():