[iOS] Fix crash when a nil URL is loaded

NSDictionary doesn't support nil values, they must be objects, so use NSNull
instead, which is the kosher way to do it.

ios/sdk/src/JitsiMeetView.m

@@ -210,7 +210,7 @@ static NSMapTable<NSString *, JitsiMeetView *> *views;
 - (void)loadURLObject:(NSDictionary *)urlObject {
     NSDictionary *props = @{
         @"externalAPIScope": externalAPIScope,
-        @"url": urlObject,
+        @"url": urlObject ?: [NSNull null],
         @"welcomePageEnabled": @(self.welcomePageEnabled)
     };