Adds support for empty per spec

[hanrelan]

This adds support for the empty keyword per spec

I think there's an issue where if a reserved word appears anywhere in the name of a var, it causes a problem. But this seems to occur for nil as well, so I think it's unrelated to this PR

[edgurgel]

I think there's an issue where if a reserved word appears anywhere in the name of a var, it causes a problem. But this seems to occur for nil as well, so I think it's unrelated to this PR

Yeah you are right! I will create an issue for this. We might need to rename another variable on the integration test for case {{ empty_string | truncatewords: 3, "" }}