[deploy-mg][mx] Fix generate golden config db failed in mx with OS ve…

…rsion doesn't support dhcp_server (sonic-net#16086)

What is the motivation for this PR?
Golden config db var has been changed from json like string to dict by this RP sonic-net#15922
But for mx with OS version doesn't support dhcp_server, it would still generate empty json like string, which would cause failure

How did you do it?
Change empty golden config generated for mx doesn't support dhcp_server

How did you verify/test it?
Deploy-mg

ansible/library/generate_golden_config_db.py

@@ -66,7 +66,7 @@ def generate_mx_golden_config_db(self):
         # Generate FEATURE table from init_cfg.ini
         ori_config_db = json.loads(out)
         if "FEATURE" not in ori_config_db or "dhcp_server" not in ori_config_db["FEATURE"]:
-            return "{}"
+            return {}
 
         ori_config_db["FEATURE"]["dhcp_server"]["state"] = "enabled"
         gold_config_db = {