Don't show UNLOCK button when there are no users.

Alternate lock screen shows UNLOCK button in the middle of an
empty screen before any user has been created. See the screenshot
in the bug.

This fixes it. The approach is similar to the check in
KeyguardViewMediator.handleHide().

Bug: 27109342
Change-Id: I25536364b2c456d6fd55c79de7f25d2b0667e26f