adjust journey test expectations

Author: Byron

tests/snapshots/plumbing/no-repo/pack/index/create/output-dir-restore-as-json-success

@@ -55,5 +55,5 @@
   "object_hash": "Sha1",
   "index_path": ""
   "data_path": ""
-  "keep_path": ""
+  "keep_path": null
 }