services(nim): fix function call

[vipyne]

Please describe the changes in your PR. If it is addressing an issue, please reference that as well.

function calling with NIM (examples/foundational/14j-function-calling-nim.py) gives the error:

2024-12-16 22:51:02.205 | ERROR    | pipecat.pipeline.task:_handle_upstream_frame:64 - Error running app: ErrorFrame#0(error: Error code: 400 - {'type': 'about:blank', 'status': 400, 'title': 'Bad Request', 'detail': "[{'type': 'string_too_short', 'loc': ('body', 'messages', 5, 'content'), 'msg': 'String should have at least 1 character', 'input': '', 'ctx': {'min_length': 1}}]"}, fatal: False)

I tracked it down to this line.

there may be a more elegant way to solve this, but this PR does the trick for me.

[markbackman]

@vipyne rather than change NimLLMService due to a Grok requirement, let's update the GrokLLMService. I've made that change here: #877.

In that PR, I made the example changes you've recommended here.

[vipyne]

closed in favor of #877