common: Add test to check that a required field that is allowed to be null passes validation

[dilyand]

If we have a schema like:

{
  (...),
  "properties": {
    "a": { "type": ["boolean", "null"] }
  },
  "additionalProperties": false,
  "required": [ "a" ]
}

we should expect a payload like:

{
  "a": null
}

to pass validation.

This test demonstrates the behaviour with the client_session context, which has a property like that and is one of our OOB contexts (ie, users have no control over it).