Allow to use LegacyNullToEmptyString on USVString

This is needed for the CSSOM spec, as that allows to use USVString as
the underlying string type. See
w3c/csswg-drafts#5010.

index.bs

@@ -10377,13 +10377,13 @@ for the specific requirements that the use of
 
 <h4 id="LegacyNullToEmptyString" extended-attribute lt="LegacyNullToEmptyString" oldids="TreatNullAs">[LegacyNullToEmptyString]</h4>
 
-If the [{{LegacyNullToEmptyString}}] [=extended attribute=] appears on the {{DOMString}} type, it creates a new
-IDL type such that that when an ECMAScript <emu-val>null</emu-val> is converted to the IDL type, it
-will be handled differently from its default handling. Instead of being stringified to
+If the [{{LegacyNullToEmptyString}}] [=extended attribute=] appears on the {{DOMString}} or {{USVString}} type,
+it creates a new IDL type such that that when an ECMAScript <emu-val>null</emu-val> is converted to the IDL type,
+it will be handled differently from its default handling. Instead of being stringified to
 "<code>null</code>", which is the default, it will be converted to the empty string.
 
 The [{{LegacyNullToEmptyString}}] extended attribute must not be
-[=extended attribute associated with|associated with=] a type that is not {{DOMString}}.
+[=extended attribute associated with|associated with=] a type that is not {{DOMString}} or {{USVString}}.
 
 Note: This means that even <code class="idl">DOMString?</code> must not use [{{LegacyNullToEmptyString}}], since
 <emu-val>null</emu-val> is a valid value of that type.