[Physical Keyboard] Null check before using layout descriptor

If descriptor is null, pass null to LayoutSelectedCallback.onSelected to
get default layout preview image.

Bug: 396006944
Flag: com.android.settings.keyboard.keyboard_and_touchpad_a11y_new_page_enabled
Test: atest
packages/apps/Settings/tests/robotests/src/com/android/settings/inputmethod/

Change-Id: I6b454a5f0b95c571cac15258977aab5ee12d4327