Resolve Promise to YES for Linking.openURL (#24955)

Summary:
The `Linking.openURL()` method is supposed to resolve to a `true` `Promise` when the URL was properly open. However, in iOS, the `Promise` was resolving to `nil`. So I fixed this issue by making it resolve to `YES` (`true`), just like on Android.

## Changelog

[iOS] [Fixed] - Fix return value of `Linking.openURL()`
Pull Request resolved: https://github.com/facebook/react-native/pull/24955

Differential Revision: D15411994

Pulled By: cpojer

fbshipit-source-id: 90bdd6cfae8d69d7a239a0b4c84d75859e293b32

Libraries/LinkingIOS/RCTLinkingManager.m

@@ -95,7 +95,7 @@ RCT_EXPORT_METHOD(openURL:(NSURL *)URL
   if (@available(iOS 10.0, *)) {
     [RCTSharedApplication() openURL:URL options:@{} completionHandler:^(BOOL success) {
       if (success) {
-        resolve(nil);
+        resolve(@YES);
       } else {
         reject(RCTErrorUnspecified, [NSString stringWithFormat:@"Unable to open URL: %@", URL], nil);
       }
@@ -103,7 +103,7 @@ RCT_EXPORT_METHOD(openURL:(NSURL *)URL
   } else {
     BOOL opened = [RCTSharedApplication() openURL:URL];
     if (opened) {
-      resolve(nil);
+      resolve(@YES);
     } else {
       reject(RCTErrorUnspecified, [NSString stringWithFormat:@"Unable to open URL: %@", URL], nil);
     }