'null' not getting interpolated as empty string

[squarestar]

In this mixin:

@mixin Rounded($radius1, $direction: null, $radius2: false) {
    $corner: null;
    @if $direction == TL { $corner: top-left-; }
    @if $direction == TR { $corner: top-right-; }
    @if $direction == BL { $corner: bottom-left-; }
    @if $direction == BR { $corner: bottom-right-; }
    @if $radius2 {
        -webkit-border-#{$corner}radius: $radius1 $radius2;
                border-#{$corner}radius: $radius1 $radius2;
    } @else {
        -webkit-border-#{$corner}radius: $radius1;
                border-#{$corner}radius: $radius1;
    }
}

The value of $corner is written as 'null' instead of an empty string, as indicated by the manual:

Null values are treated as empty strings for string interpolation:
$value: null;
p:before {
content: "I ate #{$value} pies!";
}

[leafo]

Just documenting some more cases of null not working:

$list: null;
.div {
    one: null;
    two: a $list, b;
}

scss:

.div {
  one: null;
  two: a, b; }

scssphp:

.div {
  one: null;
  two: a null, b; }