Strips valid null values from mixin arguments

[2metres]

An example of a mixin that takes 'null' as an argument from bourbon https://github.com/thoughtbot/bourbon/commit/49c93793285d409b488b0303dc8a8d23a16296bb

// Before prettysass
.toolbar {
  @include position(absolute, 0px 0px null 0px);
}
// After prettysass
.toolbar {
  @include position(absolute, 0px 0px 0px);
}