gemini: Fix "invalid argument" error when request contains no tools (#28747)

When we do not have any tools, we want to set the `tools` field to
`None`

Release Notes:

- Fixed an issue where Gemini requests would sometimes return a Bad
Request ("Invalid argument...")
e1c42315dcBennet Bo Fenner committed on 4/15/2025, 7:57:54 AM· committed by GitHubparentcfc848d
1 file changedLine totals unavailable