[css-color-adjust-1] Return no-preference, not null.

Author: tabatkins

css-color-adjust-1/Overview.bs

@@ -175,7 +175,7 @@ Finding the Used Color Scheme {#color-scheme-processing}
 			in specified order,
 			that is supported by the user agent.
 
-		4. Otherwise, return null.
+		4. Otherwise, return <css>no-preference</css>.
 	</div>
 
 	For each element,