fix incorrect role labels for migrated issues and comments (#22914)
Fix #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.