Fix FIRMessagingExtensionHelper crash when attachment == nil (#4689)

Author: maksymmalyhin

Firebase/Messaging/FIRMessagingExtensionHelper.m

@@ -46,7 +46,9 @@ - (void)populateNotificationContent:(UNMutableNotificationContent *)content
   if (attachmentURL) {
     [self loadAttachmentForURL:attachmentURL
              completionHandler:^(UNNotificationAttachment *attachment) {
-               self.bestAttemptContent.attachments = @[ attachment ];
+               if (attachment != nil) {
+                 self.bestAttemptContent.attachments = @[ attachment ];
+               }
                [self deliverNotification];
              }];
   } else {