Fix a NPE when clicking encryption interstitial header

Pref tree handler cannot assume preference has non-null key.

Change-Id: I790e75f3b4c888e2a48b297bea89cda0b8dbaabd
Fix: 35855587
Test: runtest --path tests/app/src/com/android/settings/EncryptionInterstitialTest.java

src/com/android/settings/EncryptionInterstitial.java

@@ -196,6 +196,9 @@ public class EncryptionInterstitial extends SettingsActivity {
         @Override
         public boolean onPreferenceTreeClick(Preference preference) {
             final String key = preference.getKey();
+            if (key == null) {
+                return super.onPreferenceTreeClick(preference);
+            }
             if (key.equals(KEY_ENCRYPT_REQUIRE_PASSWORD)) {
                 final boolean accEn = AccessibilityManager.getInstance(getActivity()).isEnabled();
                 if (accEn && !mPasswordRequired) {