You don't have to check the IDisposable object for null. using will not throw an exception and Dispose() will not be called:
DisposableObject TryOpenFile()
{
return null;
}
// disposable is null here, but this does not throw an exception
using (var disposable = TryOpenFile())
{
// this will throw a NullReferenceException because disposable is null
disposable.DoSomething();
if(disposable != null)
{
// here we are safe because disposable has been checked for null
disposable.DoSomething();
}
}