Correctly register storeDirectory in l10n cache

Correctly register storeDirectory in l10n cache

e4468a1d6b6 made LocalisationCache a service and refactored a bunch of
setup code. In doing so, when processing 'storeDirectory' from
$wgLocalisationCacheConf, it accidentally started treating empty
non-null values (such as the default "false") as storage paths instead
of meaning "fall back to $wgCacheDirectory". This would have broken all
config that used file store for LocalisationCache and did not specify
'storeDirectory'.

Bug: T231183
Change-Id: I9ff16be628996b202599e3bb2feed088af03775f