do not emit empty media queries

src/output.rs

@@ -155,6 +155,9 @@ impl Css {
                 }
                 Toplevel::AtRule(r) => match r {
                     AtRule::Unknown(u) => {
+                        if u.body.is_empty() {
+                            continue;
+                        }
                         if u.params.is_empty() {
                             writeln!(buf, "{}@{} {{", padding, u.name)?;
                         } else {

tests/media.rs

@@ -21,3 +21,4 @@ test!(
     "@foo {\n  a {\n    color: red;\n  }\n}\n"
 );
 test!(unknown_at_rule_no_selector, "@foo {\n  color: red;\n}\n");
+test!(empty, "@media (min-width: 2px) {}", "");