Fixed an issue when INotifyCollectionChanged.CollectionChanged event stripped by Xamarin linker.

Fix to the issue https://github.com/MvvmCross/MvvmCross-Binaries/issues/3

The code is stupid, but it do the job.
In case we assume the linker would be smarter in the future to understand that collection is always null maybe the next trick could be used:
```c#
//INotifyCollectionChanged collection = null;
INotifyCollectionChanged collection = new object() as INotifyCollectionChanged;
```