fix(hub): Make startTimestamp parameter optional in NoOpHub's startSpan method

Updated the startSpan method in the NoOpHub class to ensure the startTimestamp parameter is optional, enhancing flexibility in span creation. This change aligns with the asynchronous nature of span handling introduced in previous commits.

Author: buenaflor

packages/dart/lib/src/noop_hub.dart

@@ -178,15 +178,15 @@ class NoOpHub implements Hub {
       String name, Future<T> Function(SentrySpanV2 span) callback,
       {Map<String, SentryAttribute>? attributes,
       SentrySpanV2? parentSpan = const UnsetSentrySpanV2(),
-      DateTime? startTimestamp}) {
+      DateTime? startTimestamp}) async {
     return callback(NoOpSentrySpanV2.instance);
   }
 
   @override
   T startSpanSync<T>(String name, T Function(SentrySpanV2 span) callback,
       {Map<String, SentryAttribute>? attributes,
       SentrySpanV2? parentSpan = const UnsetSentrySpanV2(),
-      DateTime? startTimestamp}) {
+      DateTime? startTimestamp}) async {
     return callback(NoOpSentrySpanV2.instance);
   }