fix(js/plugins/compat-oai): Content is not empty if tool_calls is []

[ifielker]

This fixes #5206

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[gemini-code-assist]

Code Review

This pull request addresses a bug in the OpenAI compatibility layer where message content was being ignored when tool_calls was provided as an empty array. The fix ensures that toolRequestParts is only processed if it contains elements, and a new test case has been added to verify this behavior. Feedback indicates that the same logic should be applied to the streaming response handler (fromOpenAIChunkChoice) to maintain consistency and prevent similar issues in streaming scenarios.