I can't believe I never noticed this before.
If I type "0" (zero, sans quotes) in a private chat message and press enter, nothing shows up. It's just blank.
-0, 0-, 0 and any other character shows up. Any other number shows up. 0 by itself, nothing.
The person I was chatting with couldn't do it either. Have since tested with others as well.
0
It's just part of the checking whether a message was sent. The 0 enumerates to no message!