Fix 'dumpsys backup'

"for (Type foo : mSetOfType)" doesn't work when mSetOfType is null.

Change-Id: Iac667c7309f99b12d3147830e166b542b29cba5f
diff --git a/services/java/com/android/server/BackupManagerService.java b/services/java/com/android/server/BackupManagerService.java
index ecf1a1d..91dfaf3 100644
--- a/services/java/com/android/server/BackupManagerService.java
+++ b/services/java/com/android/server/BackupManagerService.java
@@ -2567,8 +2567,10 @@
 
             pw.println("Ancestral packages: "
                     + (mAncestralPackages == null ? "none" : mAncestralPackages.size()));
-            for (String pkg : mAncestralPackages) {
-                pw.println("    " + pkg);
+            if (mAncestralPackages != null) {
+                for (String pkg : mAncestralPackages) {
+                    pw.println("    " + pkg);
+                }
             }
 
             pw.println("Ever backed up: " + mEverStoredApps.size());