fix(app-distribution, ios): avoid crash when releaseNotes is nil (#5667)

Author: YukiKitagata

packages/app-distribution/ios/RNFBAppDistribution/RNFBAppDistributionModule.m

@@ -96,12 +96,10 @@ - (dispatch_queue_t)methodQueue {
     resolve(@{
       @"displayVersion" : release.displayVersion,
       @"buildVersion" : release.buildVersion,
-      @"releaseNotes" : release.releaseNotes,
+      @"releaseNotes" : release.releaseNotes == nil ? [NSNull null] : release.releaseNotes,
       @"isExpired" : [NSNumber numberWithBool:release.isExpired],
       @"downloadURL" : release.downloadURL
     });
-
-    resolve([NSNull null]);  // FIXME read the result into a map and return it!
   }];
 }