Fix type check for OpQuery.filter

Just using ht.TListOf as a type check doesn't work correctly. The
function must be called with the expected item type. In this specific
case TListOf was always called with the filter as a value, and the
result of that call evaluated to truth. Since filters can be quite
complex there's no check yet, and therefore just “TList” is used.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>