$foo: null;

@mixin bar($bar:null) {
  aa: type-of($bar);
  ab: type-of(unqoute($bar));
}

@mixin baz($baz) {
  ba: type-of($baz);
  bb: type-of(unqoute($baz));
}

foo {
  a: type-of('');
  b: type-of(unqoute(''));
  c: type-of(null);
  d: type-of(unquote(null));
  e: type-of($foo);
  f: type-of(unquote($foo));
  @include bar;
  @include baz(null);
}