Add a small prefilter on plpage to avoid notices when using {if $machin}

Add a small prefilter on plpage to avoid notices when using {if $machin}

Just write {if t($machin)} to avoid the notice.

WARN: only works with pattern {if t(...)}, don't try {if t(...) || t(...)}
(or improve the regexp :).

Signed-off-by: Florent Bruneau <florent.bruneau@polytechnique.org>