Fix EntityIdLabelFormatter::lookupEntityLabel

So that it actually returns false (and not null) if we
can't find a label (but the item exists).

See also my comment on:
https://gerrit.wikimedia.org/r/#/c/171530/21/lib/includes/formatters/EntityIdLabelFormatter.php

Bug: 73580
Change-Id: Iecddbbb166b544a779317c97cc54198aff9ceab2