If TIsLength is applied to a non-container item, it will fail (type
error) due to invalid application of len(). Since this can happen on
user-supplied data, we add an explicit TList/TTuple check (the TTuple
test is a new one).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>