Symptom, you create a signature and when you insert it into your email, you get a null character. Chances are in your signature you used a % sign. The percent sign is a special character to insert other special characters like space. e.g. %20 means space or %2A means *. So when you add a % sign on it's own, the Mail app doesn't know what to do with it and does something horrible like insert (null) in your email.

To add a percent sign in your signature you should put %25 which means add a percent sign.

Wikipedia has an excellent article on percent-encoding.

http://en.wikipedia.org/wiki/Percent-encoding

Rule of thumb: probably avoid using the percent sign in your signatures unless you really need to.