fix(autodoc): handle IndexError in get_type_comment for empty source

[jlaportebot]

Summary

Fixes #14345

When inspect.getsource returns whitespace-only source (e.g. "\n"), which can happen on Python 3.14+ for classes with a custom __module__ attribute, ast.parse produces an empty Module body. Accessing module.body[0] then raises IndexError, which propagates as an autodoc warning:

error while formatting arguments for pytest.PytestUnraisableExceptionWarning: list index out of range [autodoc]

Fix

Add IndexError to the existing except SyntaxError clause in get_type_comment() so it gracefully returns None instead of propagating the error.

This is a one-line change that follows the same pattern as the existing OSError/TypeError/SyntaxError handlers — when source parsing fails for any reason, return None and let autodoc fall back to other introspection methods.

Test Plan

• Verified that get_type_comment with mocked getsource returning "\n" now returns None instead of raising IndexError
• Verified that get_type_comment with valid source still works correctly
• Verified that ast.parse("\n").body is [] (confirming the root cause)