unquoted, empty strings are null

src/builtin/string.rs

@@ -55,7 +55,6 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
         Box::new(|args, _| {
             max_args!(args, 1);
             match arg!(args, 0, "string") {
-                Value::Ident(i, _) if i.is_empty() => Ok(Value::Null),
                 i @ Value::Ident(..) => Ok(i.unquote()),
                 v => Err(format!("$string: {} is not a string.", v).into()),
             }

src/value/mod.rs

@@ -99,7 +99,11 @@ impl Display for Value {
 
 impl Value {
     pub fn is_null(&self) -> bool {
-        self == &Value::Null
+        match self {
+            &Value::Null => true,
+            Value::Ident(i, QuoteKind::None) if i.is_empty()  => true,
+            _ => false
+        }
     }
 
     pub fn is_true(&self) -> SassResult<bool> {