Allow null input in StepRequestBody for Continuous Chat Mode

[hunteraraujo]

Background

This pull request updates the StepRequestBody class to allow for a null value in the input field. The change is critical for supporting a "Continuous Chat Mode" where the absence of an input message should be allowed to automatically trigger the next step in the agent conversation.

Changes 🏗️

Updated the StepRequestBody class to set the default value of input to None instead of ..., thus allowing null values.

PR Quality Scorecard ✨

• Have you used the PR description template?   +2 pts
• Is your pull request atomic, focusing on a single change?   +5 pts
• Have you linked the GitHub issue(s) that this PR addresses?   +5 pts
• Have you documented your changes clearly and comprehensively?   +5 pts
• Have you changed or added a feature?   -4 pts
  • Have you added/updated corresponding documentation?   +4 pts
  • Have you added/updated corresponding integration tests?   +5 pts
• Have you changed the behavior of Auto-GPT?   -5 pts
  • Have you also run agbenchmark to verify that these changes do not regress performance?   +10 pts

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