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

roomserver/storage/sqlite3/state_block_table.go

@@ -86,7 +86,7 @@ func (s *stateBlockStatements) BulkInsertStateData(
 	entries types.StateEntries,
 ) (id types.StateBlockNID, err error) {
 	entries = entries[:util.SortAndUnique(entries)]
-	var nids types.EventNIDs
+	nids := types.EventNIDs{} // zero slice to not store 'null' in the DB
 	for _, e := range entries {
 		nids = append(nids, e.EventNID)
 	}