one_not_null= treats missing keys as null valued

[pivotaljohn]

(From #707 synthesis: one_not_null= should treat the absence of a key as equivalent to present and null.)

---

With this schema:

#@data/values-schema
---
#@schema/type any=True
#@schema/validation one_not_null=["rootdirectory", "path"]
filesystem:
  rootdirectory: ""
  path: ""

this fails:

#@data/values
---
filesystem:
  path: ""

Instead, it should succeed, treating rootdirectory as missing to be equivalent to it being present with a null value.