Validator: let 'null' be used as a explicit value of a property

Previously 'null' was treated like undefined value and 'deafaultValue'
from metadata was used when available. Now 'null' can be used as an
explicit value.

E.g.:
  metadata = {
    a: {
      defaultValue: 1
    }
  };

Previously:

  var obj = validator.validateCompleteness(metadata, {a: null});
  obj.a === 1    // true
  obj.a === null // false

Now:

  var obj = validator.validateCompleteness(metadata, {a: null});
  obj.a === 1    // false
  obj.a === null // true