fix: replace empty user text message as 'blank text'

[poshinchen]

Description

When user sends empty string as user prompt, the model will fail with empty message exception:

An error occurred: An error occurred (ValidationException) when calling the ConverseStream operation: The text field in the ContentBlock object at messages.xxx.content.0 is blank. Add text to the text field, and try again.

Therefore updating the user text message to [blank text] when it's empty.

Related Issues

#514

Documentation PR

N/A

Type of Change

Bug fix

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• I ran hatch run prepare
• I compared the behaviors before and after the change

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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[awsarron]

If a user intentionally sends a blank string (or None), I think we should either pass that through or throw early.

This feels like model provider specific logic too. Do models other than Bedrock fail in this scenario?

[poshinchen]

We plan to fix it in a better way in our next major release.