Handle case where new keys are added to JSON objects (--format json)

[simonw]

Right now if the first JSON object is missing keys that are present in the second JSON object the script crashes.

[simonw]

I'm going to fix this by having the load_json() function normalize the JSON it loads it so any missing keys from any of the objects become keys against null.

[simonw]

I'll close this issue once I've seen my production deployment of this fix work correctly for a while.