Null is the newest in a line of hoaxes created by the Minecraft fanbase. It supposedly leaves signs with the word "null" on them. This is not in any way creepy or unnatural, it's due to a parsing failure by Java. Place down a sign in 1.7, load the world in 1.8, and then load it back in 1.7 again and any unoccupied spaces in your signs will change to "null" (may take a few tries).
Null's evidence is still invalid, its existence unknown, Mojang did not give an answer about it. But it is possible that this is a flaw in your game, a hacker or a virus that infected your game.