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 {