[cssom] Don't serialize empty rules (@media)

#10974 missed a spot.

Author: andruud

cssom-1/Overview.bs

@@ -1747,7 +1747,8 @@ To <dfn export>serialize a CSS rule</dfn>, perform one of the following in accor
    <li>The string "<code>@media</code>", followed by a single SPACE (U+0020).</li>
    <li>The result of performing <a>serialize a media query list</a> on rule's media query list.</li>
    <li>A single SPACE (U+0020), followed by the string "{", i.e., LEFT CURLY BRACKET (U+007B), followed by a newline.</li>
-   <li>The result of performing <a>serialize a CSS rule</a> on each rule in the rule's {{CSSGroupingRule/cssRules}} list, separated by a newline and indented by two spaces.</li>
+   <li>The result of performing <a>serialize a CSS rule</a> on each rule in the rule's {{CSSGroupingRule/cssRules}} list,
+   filtering out empty strings, indenting each item with two spaces, all joined with newline.</li>
    <li>A newline, followed by the string "}", i.e., RIGHT CURLY BRACKET (U+007D)</li>
   </ol>