Null not being treated as false for if statements

Howdy,

And into this issue trying to use Bourbon with scssphp.  Scssphp is not recognizing the null keyword passed in to the if statement as a false.  This is handled correctly with the ruby sass implementation.  I added a small fix on the lib_if to check for the null case as well as the false case.  I also added as small test to show the issue.