Fix CreateFileTool to allow empty files

[codegen-sh]

Description

This PR fixes the CreateFileTool to allow creating empty files when the content parameter is not provided. This addresses issue CG-12260.

Changes

1. Updated CreateFileInput class to make the content parameter optional with a default empty string
2. Updated the description in the CreateFileTool class to reflect that content is optional and explain how to create an empty file

Testing

The changes are minimal and only affect the parameter definition and documentation. The underlying implementation in codebase.create_file() already supports empty strings as content.

Fixes CG-12260

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