[css-counter-styles] Specify that an unspecified descriptor returns an empty string in the OM.

Author: tabatkins

css-counter-styles/Overview.bs

@@ -2023,6 +2023,8 @@ The <code>CSSCounterStyleRule</code> interface</h3>
 		<dd>
 			The remaining attributes on getting must return a <code>DOMString</code> object
 			that contains the serialization of the associated descriptor defined for the associated rule.
+			If the descriptor was not specified in the associated rule,
+			the attribute must return an empty string.
 
 			On setting, run the following steps:
 

css-counter-styles/Overview.html

@@ -2043,6 +2043,8 @@ <h3 class="heading settled heading" data-level=9.2 id=the-csscounterstylerule-in
 		<dd>
 			The remaining attributes on getting must return a <code>DOMString</code> object
 			that contains the serialization of the associated descriptor defined for the associated rule.
+			If the descriptor was not specified in the associated rule,
+			the attribute must return an empty string.
 
 <p>			On setting, run the following steps: