Fix sqlite migration issues (#1960)

* Do not store 'null' in the database for empty JSON arrays

This can cause issues, though it should be noted that the majority
of the time this will marshal/unmarshal just fine, see
https://play.golang.org/p/Doe2NZUgv7Q

* bugfix: sqlite migration should handle create events as having no 'before' snapshot

The state snapshot for any given event in the roomserver represents the state _before_
the event. For the create event, this is nothing, so the state snapshot nid should be 0.

In some cases this wasn't happening, resulting in a nice mix of possible options including:
 - A state snapshot without any state blocks `[]` or `null`.
 - A state snapshot with a single state block with a single event, the create event, causing
   a circular loop. This is incorrect as it represents the state before the event, not after.

* Add state key check

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