[iOS] Fix loading initial URL when app is closed

We must not pass nil as the URL, that will translate to null and we won't be
able to load the initial URL which is passed as the launch parameters.

ios/sdk/src/JitsiMeetView.m

  * @param urlObject - The URL to load which may identify a conference to join.
  */
 - (void)loadURLObject:(NSDictionary *)urlObject {
-    NSDictionary *props = @{
-        @"externalAPIScope": externalAPIScope,
-        @"url": urlObject ?: [NSNull null],
-        @"welcomePageEnabled": @(self.welcomePageEnabled)
-    };
+    NSMutableDictionary *props = [[NSMutableDictionary alloc] init];
+
+    [props setObject:externalAPIScope forKey:@"externalAPIScope"];
+    [props setObject:@(self.welcomePageEnabled) forKey:@"welcomePageEnabled"];
+
+    // XXX url must not be set when nil, so it appears as undefined in JS and we
+    // check the launch parameters.
+    if (urlObject) {
+        [props setObject:urlObject forKey:@"url"];
+    }
 
     if (rootView == nil) {
         rootView