[scroll-animations-1] Update the attributes of ViewTimeline startOffset and endOffset to be nullable (#13844)

Author: BorisChiou

scroll-animations-1/Overview.bs

@@ -824,8 +824,8 @@ spec:selectors-4; type:dfn; text:selector
 		interface ViewTimeline : ScrollTimeline {
 		  constructor(optional ViewTimelineOptions options = {});
 		  readonly attribute Element subject;
-		  readonly attribute CSSNumericValue startOffset;
-		  readonly attribute CSSNumericValue endOffset;
+		  readonly attribute CSSNumericValue? startOffset;
+		  readonly attribute CSSNumericValue? endOffset;
 		};
 	</pre>