Auto merge of #15225 - DexterHaslem:fix-transform-none, r=emilio

fix transform: none serialization

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As mentioned I struggled a bit figuring out the unit tests, please let me know if I missed anything or if there is a better way as usual! I will likely give the related implementing the rest of serialization for transform a shot next

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #15195 (github issue number if applicable).

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- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/style/properties/longhand/box.mako.rs

@@ -1251,6 +1251,11 @@ ${helpers.single_keyword("animation-fill-mode",
 
     impl ToCss for SpecifiedValue {
         fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
+
+            if self.0.is_empty() {
+                return dest.write_str("none")
+            }
+
             let mut first = true;
             for operation in &self.0 {
                 if !first {

tests/unit/style/properties/serialization.rs

@@ -1067,4 +1067,27 @@ mod shorthand_serialization {
             assert_eq!(s, "mandatory");
         }
     }
+
+    mod transform {
+        pub use super::*;
+
+        #[test]
+        fn should_serialize_none_correctly() {
+            use cssparser::Parser;
+            use media_queries::CSSErrorReporterTest;
+            use style::parser::ParserContext;
+            use style::properties::longhands::transform;
+            use style::stylesheets::Origin;
+
+            let mut s = String::new();
+            let url = ::servo_url::ServoUrl::parse("http://localhost").unwrap();
+            let context = ParserContext::new(Origin::Author, &url, Box::new(CSSErrorReporterTest));
+
+            let parsed = transform::parse(&context, &mut Parser::new("none")).unwrap();
+            let try_serialize = parsed.to_css(&mut s);
+
+            assert_eq!(try_serialize.is_ok(), true);
+            assert_eq!(s, "none");
+        }
+    }
 }