fix(ai-proxy): missing parsed_url nil check

[shreemaan-abhishek]

Description

The ai-proxy plugin code had missing nil check for parsed_url. This bug remained undetected by the test cases as all test cases use valid override.endpoint. While developing the plugin, this bug remained undeteced by me too because I was using self hosted openai server.

Fixes # (issue)

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)