Make the delete handlers of HTMLEditor guarantee that the point is in content node at calling `EnsureNoFollowingUnnecessaryLineBreak
The point may not be set if the selection move is prevented or not suggested.
Therefore, it should check the caret position before calling
EnsureNoFollowingUnnecessaryLineBreak. On the other hand,
HandleDeleteCollapsedSelectionAtWhiteSpaces should use Selection instead
if caret position is not suggested. It should be handled in bug 1925424.
Differential Revision: https://phabricator.services.mozilla.com/D226092
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1925331
gecko-commit: 00271a7486f0a3874bdf784d2f876e552115d285
gecko-reviewers: m_kato