Backport #22914Fix#22797.
## Reason
If a comment was migrated from other platforms, this comment may have an
original author and its poster is always not the original author. When
the `roleDescriptor` func get the poster's role descriptor for a
comment, it does not check if the comment has an original author. So the
migrated comments' original authors might be marked as incorrect roles.
Co-authored-by: Zettat123 <zettat123@gmail.com>
Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
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