When used in a boolean concept, all strings are considered to be truthy. For example:

> db.foo.aggregate([{$project: {x: {$cond: {if: "", then: "hi", else: "bye"}}}}])
{ "_id" : ObjectId("5a31839ba9fb0c07a00ff123"), "x" : "hi" }

Most programming languages treat the empty string as "falsey". I think it would be less confusing for our users if we do the same.