According to the sass docs "Variables with null values are treated as unassigned by !default"
but libsass seems to treat null values as assigned when using !default:

$background: null;
$background: #fff !default;

.test {
  background-color: $background;
}

libsass 3.02 compiles this to

.test {
   }

Ruby sass compiles this to:

.test {
  background-color: #fff;
}

http://sassmeister.com/gist/a2e8a2424c963dd72098