fix(ai-proxy): set llm variables default value to 0

[Revolyssup]

Description

When user use access log in json format and specify $llm_prompt_tokens without the ai plugin enabled. It is replaced by empty causing json invalidation.

Access log before fix

{"@timestamp": "2025-04-08T16:21:05+05:30", "client_ip": "127.0.0.1", "status": "404", "llm_prompt_tokens": }

After fix

{"@timestamp": "2025-04-08T16:21:52+05:30", "client_ip": "127.0.0.1", "status": "404", "llm_prompt_tokens": 0}

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)