Fix SystemUI crash for work challenge

We should not assume createConfirmDeviceCredentialIntent()
can always return an intent, for example, it has separate work challenge
but lock type is None, then createConfirmDeviceCredentialIntent() will
return null.

Bug: 27782917
Change-Id: I33e40f893f61e35a2e6ce732a47d651139f246a1
1 file changed