there is an isNull()-method, just use if beforehand to detect null-values. Remember: It is quite common for methods that return a boolean to be named isX() instead of getX(), it can be the source of some confusion :)
From my experience, it returns 0. If 0 isn't a possible answer, then you can just check for 0. If it is a possible answer, then you need to do as pgsandstrom suggested.