[css-cascade-5] Make CSSImportRule.layerName nullable w3c#6576 (w3c#6849)

Co-authored-by: Tab Atkins Jr <jackalmage@gmail.com>

Author: xiaochengh

css-cascade-5/Overview.bs

@@ -1879,13 +1879,14 @@ Extensions to the <code>CSSImportRule</code> interface</h3>
 
 	<pre class='idl'>
 	partial interface CSSImportRule {
-	  readonly attribute CSSOMString layerName;
+	  readonly attribute CSSOMString? layerName;
 	};
 	</pre>
 
 	Its <dfn attribute for=CSSImportRule>layerName</dfn> attribute represents
 	the [=layer name=] declared in the at-rule itself,
-	and is an empty string if the layer is anonymous.
+	and is an empty string if the layer is anonymous, 
+	or null if the at-rule does not declare a layer.
 
 <h3 id="the-csslayerblockrule-interface">
 The <code>CSSLayerBlockRule</code> interface</h3>