null header and footer allowed in HeaderViewListAdapter.

The view infos should never be null since they are created by ListView, which is
the natural and only use of this class.
However, some tests in CTS pass null pointers. Replace null by a static empty list.

http://b/issue?id=2527753

Change-Id: I9b92fa018c89007f12be899285f75130b2c8ac40