fix: treat a function call's None args as empty in sync AFC, matching async

[Yasser-Ameur]

The synchronous automatic function calling helper get_function_response_parts skips any FunctionCall whose args field is None, which happens for any tool declared with no parameters. Chat.send_message therefore hands the bare function call back to the caller instead of running the tool.

The async twin, get_function_response_parts_async, already treats None as an empty dict before invoking the tool. That fallback was added in commit e3be9af (Add Agent Platform MCP support to async generate_content) but the sync function was never updated to match, so AsyncChat.send_message and Chat.send_message diverge for zero argument tools.

This change mirrors the async fallback in the sync function, two lines, no new abstraction, so both paths behave the same way.

Added three offline tests to the existing test_get_function_response_parts.py covering None args on both the sync and async paths and empty dict args, so the regression is caught either way in the future.

I can share a small repro script alongside the tests if useful for review.

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