[cssom] Move TreatNullAs to the correct position

Author: saschanaz

cssom-1/Overview.bs

@@ -2134,7 +2134,7 @@ interface CSSStyleDeclaration {
   getter CSSOMString item(unsigned long index);
   CSSOMString getPropertyValue(CSSOMString property);
   CSSOMString getPropertyPriority(CSSOMString property);
-  [CEReactions] void setProperty(CSSOMString property, [TreatNullAs=EmptyString] CSSOMString value, [TreatNullAs=EmptyString] optional CSSOMString priority = "");
+  [CEReactions] void setProperty(CSSOMString property, [TreatNullAs=EmptyString] CSSOMString value, optional [TreatNullAs=EmptyString] CSSOMString priority = "");
   [CEReactions] CSSOMString removeProperty(CSSOMString property);
   readonly attribute CSSRule? parentRule;
   [CEReactions] attribute [TreatNullAs=EmptyString] CSSOMString cssFloat;