fix: handle empty string in ChatSession.send_message

[Hamza-nabil]

Description of the change

Update ChatSession.send_message to check for empty contents. If contents is empty a TypeError (Same behavior as GenerativeModel.generate_content)

Motivation

Fixing issue : #133

Type of change

Choose one: Bug fix

Checklist

• I have performed a self-review of my code.
• I have added detailed comments to my code where applicable.
• I have verified that my change does not break existing code.
• My PR is based on the latest changes of the main branch (if unsure, please run git pull --rebase upstream main).
• I am familiar with the Google Style Guide for the language I have coded in.
• I have read through the Contributing Guide and signed the Contributor License Agreement.

[MarkDaoust]

That sounds right. I can't think of any cases where an empty Content would be valid.
Could we move this to content_types.to_content so it applies everywhere?

[Hamza-nabil]

Thank you for your feedback ! I've updated the code as suggested.

[MarkDaoust]

Thanks!