Auto merge of #16136 - streichgeorg:counter, r=emilio

to_css of counter-increment returns none when there are no properties.

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Changed to_css in style::properties::longhands::counter_increment returns "none" if there are no properties defined.

---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #15977 (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/counters.mako.rs

@@ -269,6 +269,10 @@
 
     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 {
@@ -278,6 +282,7 @@
                 try!(serialize_identifier(&pair.0, dest));
                 try!(write!(dest, " {}", pair.1));
             }
+
             Ok(())
         }
     }