LocalisationCache: Avoid use of compact()

In PHP 7.3, compact() now raises notices if the variable is undefined, which
is something that we expect. So we can check whether the key exists instead
of bothering with compat() and suppressing warnings.

Bug: T206979
Change-Id: I612049db4debd850a2e6d10bc631d31aa17be898
(cherry picked from commit d0463178dfa09b79b3a08fee939da1beed030824)