Just like it says on the tin, when accessing the User Stream, the source node’s following data is always returned as null. I’ve tested with a couple of different user accounts and I’ve tried retweeting a tweet, faving a tweet, unfaving a tweet. I’ve tested for faving from a private account, from a public account, to a private account, to a public account. Here the relevant JSON for one example (I’ve pruned some of the extraneous data out):
"source": {
"id": 0123456789,
"created_at": "Wed Oct 21 23:53:04 +0000 2015",
"notifications": null,
"name": "##########",
"friends_count": 2,
"statuses_count": 89,
"location": null,
"followers_count": 2,
"listed_count": 1,
"protected": true,
"description": null,
"utc_offset": null,
"screen_name": "##########",
"follow_request_sent": null,
"profile_background_tile": false,
"following": null, # <--- This should be true/false
"lang": "en",
"time_zone": null
},