Fix NPE when user goes from none to swipe for lockscreen

If the user currently doesn't have a password and transitions
into another empty lockscreen (none -> swipe or swipe -> none),
there is no need to call setLockCredential.

Bug: 142701762
Test: Not yet :(
Change-Id: I553c8b30c7414775185d632660d962a73607baca
1 file changed