Add null check in FaceService#isHardwareDetected

Similar to I6c037f90691e4da2f88419c6f80994e8ae1565b0.
All clients (keyguard, settings) should already be checking isHardwareDetected()
before doing any face-related operation.

Test: load aosp_flame-eng, sysui does not crash
Fixes: 161232319

Change-Id: I2d394114073ef3df766e04430190e3430e7d1579