Check if UserState is null in stopGuestOrEphemeralUserIfBackground

- UserState may be removed from mStartedUsers if it has already been stopped

Bug: 72133858
Test: Manually create secondary user, and exit the user in SetupWizard
Change-Id: I92783f89a9d4de9a7eca81e688b4e115c2f5535a
1 file changed