[cssom-1] Clarify what happens if CSSStyleDeclaration.item(n) is out of bounds, like all the other item methods. #12532

Author: tabatkins

cssom-1/Overview.bs

@@ -2482,7 +2482,7 @@ The <dfn attribute for=CSSStyleDeclaration>length</dfn> attribute must return th
 declarations</a> in the <a for="CSSStyleDeclaration">declarations</a>.
 
 The <dfn method for=CSSStyleDeclaration>item(<var>index</var>)</dfn> method must return the
-<a for="CSS declaration">property name</a> of the <a>CSS declaration</a> at position <var>index</var>.
+<a for="CSS declaration">property name</a> of the <a>CSS declaration</a> at position <var>index</var>. If there is no <var>index</var>th object in the collection, then the method must return the empty string.
 
 The <dfn method for=CSSStyleDeclaration>getPropertyValue(<var>property</var>)</dfn> method must run these steps:
 <ol>