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

roomserver/storage/sqlite3/events_table.go

@@ -571,6 +571,9 @@ func (s *eventStatements) SelectRoomNIDsForEventNIDs(
 }
 
 func eventNIDsAsArray(eventNIDs []types.EventNID) string {
+	if eventNIDs == nil {
+		eventNIDs = []types.EventNID{} // don't store 'null' in the DB
+	}
 	b, _ := json.Marshal(eventNIDs)
 	return string(b)
 }

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)
 	}

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 {