Auto merge of #15546 - eddiequan:15209_fix_quotes_serialization_transform_none, r=Wafflespeanut

fixes quote transform: none serialization

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Adds a unit test to reproduce faulty behaviour, and serializes "none" correctly when provided an empty list.

---
<!-- 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 #15209 (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/list.mako.rs

@@ -54,6 +54,10 @@ ${helpers.predefined_type("list-style-image", "UrlOrNone", "Either::Second(None_
 
     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 pair in &self.0 {
                 if !first {