Mongoose schema to require array that can be empty
Empty arrays are created by default (see also this). The attribute required: true requires the array to have at least one element in it (source code). You can remove that attribute to get your desired behavior.
(Aside, mongoose assigns a default _id field with the type ObjectId to all schemas. Declaring it is unnecessary, and using a string is not typical, although certainly allowed.)
Edit Nov 2017: This is a candidate change in Mongoose 5. See https://github.com/Automattic/mongoose/issues/5139.