Fix LockSettingsShellCommand handling of empty old credentials

Fix a crash when the device is secure but the user did not supply
the old credential when invoking LockSettingsShellCommand

Bug: 65239740
Test: adb shell cmd lock_settings set-pin  1254 when device has password
Change-Id: Id94f26c98984c6c09865ff45daeb76307f66dffb
1 file changed