Always report non-null TetherStatesParcel

Before this change, tetherStatesParcel may be null when there is no
avaialbe tethering interface yet. TetheringManager should handle null
case. But it is annoy to check null case each time and easy to have error.
Report non-null empty TetherStatesParcel object instead.

Bug: 148874619
Test: -build, flash, boot
      -atest TetheringTests
      -OFF/ON hotspot

Change-Id: I0717e7958c9409648170498d5e76322207856de3