The serializer needs to make each mailcite starts from head of a line. However, mailcite may be a blocked `<span>`. So, its preceding `<br>` is not required from the HTML point of view, but we need to preserve it for the serializer. If we need this hack in some other places, we should make `HTMLEditUtils::GetFollowingUnnecessaryLineBreak()` aware of this special handling in a follow up bug. Differential Revision: https://phabricator.services.mozilla.com/D270784