Prevent NPE in CommentMustAsDiff if no hunk header (#15199)

Prevent NPE in CommentMustAsDiff if no hunk header (#15199)

I do not understand how this can happen or why.

There is an apparent possibility for a comment.Patch to be missing a hunk header
- this should not happen and do not understand how. But it appears to happen on
1.13 at least in some case.

This PR will simply add a new section if the cursection is empty
thus preventing the NPE.

Fix #15198

Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Lauris BH <lauris@nix.lv>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>