Bugfix in sanity check: Prevent null-dereference

lib/src/db/db.dart

Index: lib/src/db/db.dart
diff --git a/lib/src/db/db.dart b/lib/src/db/db.dart
index bf1c3f157f74c848e4777161418c79bc88915437..c417d1c288dede83eff8a4bfedd3fcedeff93dd8 100644
--- a/lib/src/db/db.dart
+++ b/lib/src/db/db.dart
@@ -315,7 +315,7 @@ class DatastoreDB {
       } else {
         partition = defaultPartition;
       }
-    } else if (partition != ancestorKey.partition) {
+    } else if (ancestorKey != null && partition != ancestorKey.partition) {
       throw new ArgumentError(
           'Ancestor queries must have the same partition in the ancestor key '
           'as the partition where the query executes in.');