Show an empty view when there is no widget available

Test: Remove all widgets from work profile using TestDpc. Observe
      empty view is shown in the full widget tray. Add back some
      widgets to work profile and observe widgets showing up in the
      full widgets tray

Bug: 179797520
Change-Id: I2e1fcb5a0014568a4674e2bcc0a3ed3c6ec5a44a