Fix a NPE in confirm lock screen.

Change-Id: Id280cd1b8781678540c9ea281b3186edd04d61f9
Fixes: 31895774
Test: make RunSettingsRoboTests
Test: manually enter correct/wrong/empty password/pin in confirm lock UI

src/com/android/settings/ConfirmLockPassword.java

@@ -319,11 +319,15 @@ public class ConfirmLockPassword extends ConfirmDeviceCredentialBaseActivity {
                 return;
             }
 
-            mPasswordEntryInputDisabler.setInputEnabled(false);
-
             final String pin = mPasswordEntry.getText().toString();
+            if (TextUtils.isEmpty(pin)) {
+                return;
+            }
+
+            mPasswordEntryInputDisabler.setInputEnabled(false);
             final boolean verifyChallenge = getActivity().getIntent().getBooleanExtra(
                     ChooseLockSettingsHelper.EXTRA_KEY_HAS_CHALLENGE, false);
+
             Intent intent = new Intent();
             if (verifyChallenge)  {
                 if (isInternalActivity()) {