outline: Do not omit an empty nodes field when marshalling to JSON

[dlipovetsky]

If an empty 'nodes' field is represented as an empty JSON array, then a client
unmarshalling from JSON does not need to allow for the possibility that the
'nodes' field is undefined. That makes it is easier for clients to operate on
the JSON.

Before:

{
  "name": "It",
  "text": "normal",
  "start": 336,
  "end": 363,
  "spec": true,
  "focused": false,
  "pending": false
}

{
  "name": "It",
  "text": "normal",
  "start": 336,
  "end": 363,
  "spec": true,
  "focused": false,
  "pending": false,
  "nodes": []
}

[dlipovetsky]

I've been working on a vscode extension that calls ginkgo outline. It is much simpler for the client to use array semantics for the nodes field. For example, the client can always iterate over nodes, and assume the iteration will exit immediately for a leaf node, where nodes has zero length. Without this change, the client must check whether nodes is defined before iterating over it.

Feedback welcome!