docs(flutter): Fix event serverName reset in samples (#12832)

Author: denrase

platform-includes/configuration/before-send/dart.mdx

@@ -1,7 +1,7 @@
 ```dart {3}
 await Sentry.init((options) {
   options.beforeSend = (event, hint) {
-    return event.copyWith(serverName: null); // Don't send server names.
+    return event.copyWith(serverName: ""); // Don't send server names.
   };
 });
 ```

platform-includes/configuration/before-send/flutter.mdx

@@ -1,7 +1,7 @@
 ```dart {3}
 await SentryFlutter.init((options) {
   options.beforeSend = (event, hint) {
-    return event.copyWith(serverName: null); // Don't send server names.
+    return event.copyWith(serverName: ""); // Don't send server names.
   };
 });
 ```