What it usually means is that someone set up a robodialing text sender incorrectly. IOW, you should be getting spam, but the spam message is blank. There are blacklisting apps that can block anonymous texts and voice calls. (You won't even know that you got the call or text, but it'll be logged, in case someone tells you that he sent you a text about something - you can look in the log and see that you did receive a text at that time, but the callerID was blocked.)