NIU Actions: Handle null default assistant

This handles the case where Settings.Secure.ASSISTANT is null, which occurs when config_defaultAssistant is not set at build time.

Bug: 195600539
Test: Manual (built locally without config_defaultAssistant)
Test: b/195681795
Change-Id: Ie8da67be5f448847c07438a25b12e8b5b2022f05
diff --git a/go/quickstep/src/com/android/quickstep/TaskOverlayFactoryGo.java b/go/quickstep/src/com/android/quickstep/TaskOverlayFactoryGo.java
index 91cab3c..99238f0 100644
--- a/go/quickstep/src/com/android/quickstep/TaskOverlayFactoryGo.java
+++ b/go/quickstep/src/com/android/quickstep/TaskOverlayFactoryGo.java
@@ -253,7 +253,11 @@
 
             String assistantPackage =
                     Settings.Secure.getString(contentResolver, Settings.Secure.ASSISTANT);
-            mNIUPackageName = assistantPackage.split("/", 2)[0];
+            if (!TextUtils.isEmpty(assistantPackage)) {
+                mNIUPackageName = assistantPackage.split("/", 2)[0];
+            } else {
+                mNIUPackageName = "";
+            }
         }
 
         protected class OverlayUICallbacksGoImpl extends OverlayUICallbacksImpl