Remove unnecessary blank? code

Since https://github.com/Shopify/liquid/commit/2efe809e11ee5db6feea5e62d6c6585471f9f8b5, Liquid::Tag#blank? defaults to false (for backwards compatibility), which means a lot of those overrides here are unnecessary (only need to overwrite if you want your tag to be blank, not if you don't).

@arthurnn