properly emit separator in join when both are under 2 length

src/builtin/list.rs

@@ -162,9 +162,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
             ) {
                 Value::Ident(s, ..) => match s.as_str() {
                     "auto" => {
-                        if list1.len() < 2 && list2.len() < 2 {
-                            ListSeparator::Space
-                        } else if list1.is_empty() {
+                        if list1.is_empty() {
                             sep2
                         } else {
                             sep1

tests/list.rs

@@ -212,6 +212,11 @@ test!(
     "a {\n  color: join((), (a, b, c));\n}\n",
     "a {\n  color: a, b, c;\n}\n"
 );
+test!(
+    join_single_comma_both,
+    "a {\n  color: join((a,), (b,));\n}\n",
+    "a {\n  color: a, b;\n}\n"
+);
 test!(bracketed_ident, "a {\n  color: [a];\n}\n");
 test!(bracketed_space_list, "a {\n  color: [a b];\n}\n");
 test!(bracketed_comma_list, "a {\n  color: [a, b];\n}\n");