<===> input.scss
$foo: null;

@mixin bar() {
   bar: $foo;
}

@mixin baz() {
   baz: $foo !important;
}

foo {
  baz: $foo;
}

bar {
  @include bar;
}

baz {
  @include baz;
}

<===> output.css
baz {
  baz: !important;
}