Fix issue with gemini functions with no parameters

[dirkbrnd]

Description

Fixes #1558

This ensures the Gemini model implementation is compatible with both functions that have parameters and without parameters.

Type of change

Please check the options that are relevant:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Model update
• Infrastructure change

Checklist

• My code follows Phidata's style guidelines and best practices
• I have performed a self-review of my code
• I have added docstrings and comments for complex logic
• My changes generate no new warnings or errors
• I have added cookbook examples for my new addition (if needed)
• I have updated requirements.txt/pyproject.toml (if needed)
• I have verified my changes in a clean environment

[manthanguptaa]

LGTM