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
(cherry picked from commit 3bf2e707455602321fa59a5d81a7e9bfa1764b20)
diff --git a/src/com/android/settings/password/ChooseLockGeneric.java b/src/com/android/settings/password/ChooseLockGeneric.java
index 4f1b050..a2885d6 100644
--- a/src/com/android/settings/password/ChooseLockGeneric.java
+++ b/src/com/android/settings/password/ChooseLockGeneric.java
@@ -760,8 +760,12 @@
             if (quality == DevicePolicyManager.PASSWORD_QUALITY_UNSPECIFIED) {
                 // Clearing of user biometrics when screen lock is cleared is done at
                 // LockSettingsService.removeBiometricsForUser().
-                mChooseLockSettingsHelper.utils().setLockCredential(
-                        LockscreenCredential.createNone(), mUserPassword, mUserId);
+                if (mUserPassword != null) {
+                    // No need to call setLockCredential if the user currently doesn't
+                    // have a password
+                    mChooseLockSettingsHelper.utils().setLockCredential(
+                            LockscreenCredential.createNone(), mUserPassword, mUserId);
+                }
                 mChooseLockSettingsHelper.utils().setLockScreenDisabled(disabled, mUserId);
                 getActivity().setResult(Activity.RESULT_OK);
                 finish();