Save empty values for nodes

Hey!

This change is needed if `gonzales-ast` is applied to `gonzales-pe` AST (which can parse SASS and LESS besides CSS). In these syntaxes `//` comments are allowed and the AST for empty comment would be `["commentSL",""]`. But if you run `('gonzales-ast').traverse` over such AST (even with no changes in `process`), it returns `["commentSL"]` and so breaks the result.

My change allows to save empty values in the AST tree.