Fix NPE when cursor is null

Fix JoinContactLoader background loader to null-check returned cursor before
wrapping in a JoinContactLoaderResult CursorWrapper. Change
JoinContactListFragment to in turn check for a null JoinContactLoaderResult.

Bug:18373971
Change-Id: Ib973d1609ba4e555a98b2b9b8c07bd7e5e4e3601