Don't print out styles if they are null

src/css.rs

@@ -36,6 +36,9 @@ impl Toplevel {
     }
 
     fn push_style(&mut self, s: Style) {
+        if s.value.is_null() {
+            return;
+        }
         if let Toplevel::RuleSet(_, entries) = self {
             entries.push(BlockEntry::Style(Box::new(s)));
         }

tests/styles.rs

@@ -130,3 +130,8 @@ test!(
     "a {\n  color:red\n}\n",
     "a {\n  color: red;\n}\n"
 );
+test!(
+    removes_null_value,
+    "a {\n  color: null;\n}\n",
+    ""
+);