[cssom] Now that empty custom props serialize their value as space, make sure serializing a decl doesn't produce two spaces for them. #9847

Author: tabatkins

cssom-1/Overview.bs

@@ -2296,7 +2296,8 @@ these steps:
  <li>Let <var>s</var> be the empty string.
  <li>Append <var>property</var> to <var>s</var>.
  <li>Append "<code>: </code>" (U+003A U+0020) to <var>s</var>.
- <li>Append <var>value</var> to <var>s</var>.
+ <li>If <var>value</var> contains any non-whitespace characters,
+    append <var>value</var> to <var>s</var>.
  <li>If the <i>important</i> flag is set, append "<code> !important</code>" (U+0020 U+0021 U+0069 U+006D U+0070 U+006F U+0072 U+0074 U+0061 U+006E
  U+0074) to <var>s</var>.
  <li>Append "<code>;</code>" (U+003B) to <var>s</var>.
@@ -3407,4 +3408,4 @@ initial version of the alternative style sheets API and canonicalization
 (now serialization) rules for CSS values.
 
  </body>
-</html>
+</html>