<===> input.scss
@function foo() { @return null; }
$foo: null;
a {
    foo: bar;
    variable: $foo;
    function: foo();
    unquote: unquote($foo);
}

b {
    variable: $foo;
    function: foo();
    unquote: unquote($foo);
}

<===> error-dart-sass
Error: $string: null is not a string.
  ,
7 |     unquote: unquote($foo);
  |              ^^^^^^^^^^^^^
  '
  input.scss 7:14  root stylesheet

<===> output.css
a {
  foo: bar;
}

<===> warning
DEPRECATION WARNING: Passing null, a non-string value, to unquote()
will be an error in future versions of Sass.
        on line 7 of input.scss
DEPRECATION WARNING: Passing null, a non-string value, to unquote()
will be an error in future versions of Sass.
        on line 13 of input.scss