Fix NPE when reading the allowlist

Bug: 351750747
Test: atest -v TarBackupReaderTest, manual test
Flag: EXEMPT bugfix
Change-Id: Icf4fe058e7d261c197e04ccad940595413bd4a99
diff --git a/services/backup/java/com/android/server/backup/utils/TarBackupReader.java b/services/backup/java/com/android/server/backup/utils/TarBackupReader.java
index 4860a27..8abbe56 100644
--- a/services/backup/java/com/android/server/backup/utils/TarBackupReader.java
+++ b/services/backup/java/com/android/server/backup/utils/TarBackupReader.java
@@ -792,10 +792,11 @@
     }
 
     private String getVToUAllowlist(Context context, int userId) {
-        return Settings.Secure.getStringForUser(
+         String allowlist = Settings.Secure.getStringForUser(
                 context.getContentResolver(),
                 Settings.Secure.V_TO_U_RESTORE_ALLOWLIST,
                 userId);
+         return (allowlist == null) ? "" : allowlist;
     }
 
     private static long extractRadix(byte[] data, int offset, int maxChars, int radix)