Don't crash on null Intent in FingerprintEnrollFindSensor

During a run of the setup wizard, a tester one time encountered a case
where the 'Accept Legal terms' screen followed by the Security screen
showed for a second time after already confirming the pattern in the
Security screen. The logs showed a crash in
FingerprintEnrollFindSensor.onActivityResult which could only have
happened if it was called with a requestCode of CONFIRM_REQUEST and a
resultCode of RESULT_OK, but a null Intent.

This isn't an expected flow, and AFAIK we haven't been able to reproduce
the original problem, but it seems reasonable to at least not crash in
settings, so this CL defends against that crash by just calling finish()
in this case, which means the caller who started the
FingerprintEnrollFindSensor will see the default result code of
Activity.RESULT_CANCELLED.

Bug: 80099085
Test: make -j RunSettingsRoboTests
Change-Id: I94401eabe295e4d5396cf7d324fbf1902dc45f6d