Consider using non-null value for "empty string"

[RReverser]

Right now various productions such as /(?:)/, /1|/, etc. can produce null values in place of sub-expressions when parsed.

It could be useful (as in just "nice") to have instead a separate node type like { type: "Empty" } so that one could always expect node.type to work whenever node is supposed to be an expression without extra check for null.