Fix crash when launching assist from power button.

Bug: 131183785
Test: adb shell settings put global power_button_long_press 5
      then longpress power button
Change-Id: I0d6c25f75efc1e0e7a04f44ba91f7603ab8f311b

packages/SystemUI/src/com/android/systemui/assist/AssistManager.java

@@ -180,6 +180,10 @@ public class AssistManager implements ConfigurationChangedReceiver {
                     ? TIMEOUT_SERVICE
                     : TIMEOUT_ACTIVITY);
         }
+
+        if (args == null) {
+            args = new Bundle();
+        }
         args.putLong(INVOCATION_TIME_MS_KEY, SystemClock.uptimeMillis());
         startAssistInternal(args, assistComponent, isService);
     }