Disallow filters with no variable in strict mode

Solution for https://github.com/Shopify/liquid/pull/475. Variable blocks must have a variable name.

@xthexder @fw42 @dylanahsmith for review.
sorry, I think I misunderstood this comment. Filter without input here means something like ```{{ "" | filter: 123 }}```, right? I was thinking ```{{ "foo" | filter }}``` (with input, but without argument).