noUnknown gives "Cannot convert undefined or null to object"

[garylesueur]

Describe the bug

When I have a nested object using the "noUnknown" operator, if the object itself is missing from the value then it still tries to enumerate the keys of the object causing an error of

"Cannot convert undefined or null to object"

To Reproduce

https://codesandbox.io/s/mutable-waterfall-7zzdg?file=/src/index.test.js

Expected behavior
I would expect it to ignore the fact that the value is not there, and pass.

Platform (please complete the following information):

• Fails in latest Chrome, Firefox and Safari

[garylesueur]

This looks like might be same issue as #854