[ai] Google gemini error if request doesn't have functions

[Hanksha]

Bug Description:

I was trying out gemini with Theia AI and I noticed it didn't work in some cases with the following error:

{"error":{"message":"{\n \"error\": {\n \"code\": 400,\n \"message\": \"Function calling config is set without function_declarations.\",\n \"status\": \"INVALID_ARGUMENT\"\n }\n}\n","code":400,"status":"Bad Request"}}

It happens for agents with no functions like universal or code completion.

Steps to Reproduce:

1. Configure your Google gemini API key
2. Set the code-completion and universal model alias to a gemini model
3. Try a code completion or chat with the universal agent
4. Check the error in the console

Additional Information

• Operating System: Windows
• Theia Version: 1.64.1

[Hanksha]

I've looked into it, if I only pass the functionCallConfig object when functionsDeclarations is not empty here, it fixes the error.