Respond with null array to consumers

BLPOP should return what the Redis protocol calls a "null array", that
is "*-1\r\n". Previously, we returned a null string, "$-1\r\n". While
this does not have any concrete effects, it's just semantically correct
to follow the Redis protocol where we can.

See https://redis.io/topics/protocol (RESP Arrays) and
https://redis.io/commands/blpop

server.go

@@ -121,7 +121,12 @@ func (s *Server) Handle(ctx context.Context, conn net.Conn) {
 						writeErr = writer.WriteError("CONS Server shutdown")
 						break ConsLoop
 					case <-ticker.C:
-						writeErr = writer.WriteBulk(nil)
+						// WriteBulkStrings() is the only method
+						// that returns what the Redis Protocol
+						// calls a "null array" (i.e. "*-1\r\n")
+						//
+						// see https://github.com/secmask/go-redisproto/issues/4
+						writeErr = writer.WriteBulkStrings(nil)
 						break ConsLoop
 					default:
 						ev, err := cons.Poll(100)