[NUI] Make we flush DisposeQueue before app start

[NUI] Make we flush DisposeQueue before app start

If we don't call DisposeQueue.Instance.Initialize(), it might not flush
the dispose queue.

Before #5806 and until currently, we didn't allow to call ProcessDisposables()
before Initialize(). It might occure memory leak if app try to create & dispose
unlimitly before Application start.

To make ensure it, let we make ProcessDisposables() API can be called even if
DisposeQueue is not initialized.

Signed-off-by: Eunki, Hong <eunkiki.hong@samsung.com>