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_snapshot_table.go

@@ -87,6 +87,9 @@ func prepareStateSnapshotTable(db *sql.DB) (tables.StateSnapshot, error) {
 func (s *stateSnapshotStatements) InsertState(
 	ctx context.Context, txn *sql.Tx, roomNID types.RoomNID, stateBlockNIDs types.StateBlockNIDs,
 ) (stateNID types.StateSnapshotNID, err error) {
+	if stateBlockNIDs == nil {
+		stateBlockNIDs = []types.StateBlockNID{} // zero slice to not store 'null' in the DB
+	}
 	stateBlockNIDs = stateBlockNIDs[:util.SortAndUnique(stateBlockNIDs)]
 	stateBlockNIDsJSON, err := json.Marshal(stateBlockNIDs)
 	if err != nil {