unquote(null) returns a string instead of null

[xzyfer]

foo {
  a: type-of(null);
  b: type-of(unqoute(null)); }

Ruby Sass

foo {
  a: null;
  b: null; }

Libsass

foo {
  a: null;
  b: string; }

This in itself is not a regression, but due to other changes surrounding generating CSS output this causing regressions - #1106 (comment)

Specs added sass/sass-spec#335

[xzyfer]

Fixed by #1125