prevent fatal crashes when launching App Clips (#47000)

Summary:
App Clips are full of bugs. One such bug is how launching an App Clip from Test Flight (and perhaps other systems) will cause the user activity to be of type `NSUserActivityTypeBrowsingWeb` but with a nullish `userActivity.webpageURL` (even though it's typed as never being nullish in this mode).

## Changelog:

[IOS] [FIXED] - Fix launching App Clips with nullish URLs.

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Pull Request resolved: #47000

Test Plan:
The following was failing cryptically and required extensive debugging to get launching.

https://github.com/user-attachments/assets/ffd26297-b20b-45a2-97cc-35a48b740cb2

Reviewed By: cipolleschi

Differential Revision: D64372170

Pulled By: lunaleaps

fbshipit-source-id: 388f05530f3bccdd9b446382931f127421bc8a27

packages/react-native/Libraries/LinkingIOS/RCTLinkingManager.mm

@@ -74,7 +74,8 @@ + (BOOL)application:(UIApplication *)application
     continueUserActivity:(NSUserActivity *)userActivity
       restorationHandler:(nonnull void (^)(NSArray<id<UIUserActivityRestoring>> *_Nullable))restorationHandler
 {
-  if ([userActivity.activityType isEqualToString:NSUserActivityTypeBrowsingWeb]) {
+  // This can be nullish when launching an App Clip.
+  if ([userActivity.activityType isEqualToString:NSUserActivityTypeBrowsingWeb] && userActivity.webpageURL != nil) {
     NSDictionary *payload = @{@"url" : userActivity.webpageURL.absoluteString};
     [[NSNotificationCenter defaultCenter] postNotificationName:kOpenURLNotification object:self userInfo:payload];
   }