The message cache is originally meant for mw.messages in JS,
which expects non-existent messages to be cleanly omitted.
There is minimal server-side and client-side handling in place for this.
For LESS, however, we were assuming that the blob is complete,
thus not feeding anything to the LESS compiler, thus leading to
a run-time failure where the LESS file can't be parsed at all
due to an undeclared variable.
This could happen sometimes during development or after upgrading
a wiki with a stale LocalisationCache that is still being updated
at that time.
Bug: T267785
Change-Id: I60ff4eb7dce1fee56470acc177afd29ee14b764f
8ad97d7c32