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

@@ -117,14 +117,17 @@ from ansible.module_utils.six import iteritems
 from ansible.module_utils.pycompat24 import get_exception
 
 def _external_ids_to_dict(text):
-    d = {}
+    if not text:
+        return None
+    else:
+        d = {}
 
-    for l in text.splitlines():
-        if l:
-            k, v = l.split('=')
-            d[k] = v
+        for l in text.splitlines():
+            if l:
+                k, v = l.split('=')
+                d[k] = v
 
-    return d
+        return d
 
 def map_obj_to_commands(want, have, module):
     commands = list()