Fix 4399 ASTSafetyError on comment in return type annotation

[dankrzeminski32]

Description

Resolves #4399

Added an extra check before adding commas to the elements in the body of a bracket split.

The check ensures that there is more than just one non-comment item in the list of return elements. It only performs this check for a bracket split in return annotations as it's non-problematic for parameter annotations.

The extra comment was causing the ASTSafetyError (it led to the return type being interpreted as a tuple).

Checklist - did you ...

• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?

[hauntsaninja]

Thank you for the PR!

I have a slightly different fix in mind. We should try and find a leaf that actually has the appropriate parent set. This will fix a similar crash in another case, see:

def foo() -> (
    # comment inside parenthesised new union return type
    int | str | bytes
):
    ...

I opened a PR with what I had in mind at #4453. I'd be grateful if you had time to take a look and see if it makes sense to you!