Null values in lists are not removed when interpolated in a string

[natecavanaugh]

The bug happens in libsass 3.2.5.
SCSS:

.div{
  $foo: 1, null, 2, null, 3;

  content: "#{$foo}";
}

Ruby Sass output:

.div {
  content: "1, 2, 3";
}

libsass output:

.div {
  content: "1, , 2, , 3";
}

I looked through the existing issues, and saw a few issues/fixes around lists of strings being interpolated improperly, but not sure if they also happened to fix this.
Thought I'd submit this, just in case it hadn't been caught :)

[mgreter]

This seems just another specific detail we haven't caugth yet!

[xzyfer]

Fixed via #1389

[natecavanaugh]

Awesome, thank you! :)