Change baseURL type to be nullable

Author: zeesuyeon

cssom-1/Overview.bs

@@ -1091,7 +1091,7 @@ interface CSSStyleSheet : StyleSheet {
 };
 
 dictionary CSSStyleSheetInit {
-  DOMString baseURL = null;
+  DOMString? baseURL = null;
   (MediaList or DOMString) media = "";
   boolean disabled = false;
 };