[css-values-5] Allow explicit null-namespace in attr-name (#11359)

Otherwise we're making a non-backwards-compatible change against
content:attr() from css-content [1].

[1] https://drafts.csswg.org/css-content/#strings

Author: andruud

css-values-5/Overview.bs

@@ -1556,7 +1556,7 @@ Ian's proposal:
 	<pre class=prod>
 		attr() = attr( <<attr-name>> <<attr-type>>? , <<declaration-value>>?)
 
-		<dfn>&lt;attr-name></dfn> = [ <<ident-token>> '|' ]? <<ident-token>>
+		<dfn>&lt;attr-name></dfn> = [ <<ident-token>>? '|' ]? <<ident-token>>
 		<dfn>&lt;attr-type></dfn> = type( <<syntax>> ) | string | <<attr-unit>>
 	</pre>