Handle edge-case where no focusables exist

cluster navigation was previously crashing if there was
nothing focusable..

Bug: 37438383
Test: Added CTS: ContentPaneFocusTest#testNoFocusablesInContent
      Ran against support-v7-demos Toolbar demos to verify
      crash didn't occur

Change-Id: Ia1382cec138a948d080aeded4d38735983df2152