I had the issue that every time I turned off the phone it would send a text message to one person. I took it to Verizon and they did a hard reset and said it was fixed. I knew better and tried it again in the store. Needless to say it sent the blank text again. They immediately gave me a new phone... no questions asked.
I posed the question on this forum and was told that if there is a blank draft in your saved messages it will send it at times. I would make sure that you do not have a blank text sitting in your messages before I return the phone.