`AlignStateAtSelection` class is instantiated outside of editor class so that
we shouldn't make each user guarantee that there is selection range
(fortunately, the putting off cost is really low).
And as far as I tested, Blink and WebKit does not throw exception when
`Document.qeuryCommand*("justify*")` is called without selection ranges.
So, this patch also prevents exception in this situation.
Differential Revision: https://phabricator.services.mozilla.com/D68755
--HG--
extra : moz-landing-system : lando
Masayuki Nakano