more robustly emit empty arglists

src/value/mod.rs

@@ -185,6 +185,9 @@ impl Value {
             Value::True => Cow::const_str("true"),
             Value::False => Cow::const_str("false"),
             Value::Null => Cow::const_str(""),
+            Value::ArgList(args) if args.is_empty() => {
+                return Err(("() isn't a valid CSS value.", span).into());
+            }
             Value::ArgList(args) => Cow::owned(
                 args.iter()
                     .filter(|x| !x.is_null())
@@ -287,6 +290,7 @@ impl Value {
                     .join(", ")
             )),
             Value::Dimension(num, unit) => Cow::owned(format!("{}{}", num, unit)),
+            Value::ArgList(args) if args.is_empty() => Cow::const_str("()"),
             Value::ArgList(args) => Cow::owned(
                 args.iter()
                     .filter(|x| !x.is_null())

tests/arglist.rs

@@ -21,7 +21,6 @@ test!(
         @each $number in $numbers {
             $sum: $sum + $number;
         }
-    
         @return $sum;
     }
     
@@ -30,3 +29,25 @@ test!(
     }",
     "a {\n  width: 180px;\n}\n"
 );
+error!(
+    emit_empty_arglist,
+    "@function foo($a...) {
+        @return $a;
+    }
+    
+    a {
+        color: foo();
+    }",
+    "Error: () isn't a valid CSS value."
+);
+test!(
+    inspect_empty_arglist,
+    "@function foo($a...) {
+        @return inspect($a);
+    }
+    
+    a {
+        color: foo();
+    }",
+    "a {\n  color: ();\n}\n"
+);