Validator.getErrors() throws an error on undefined fields

[XX-NF-XX]

Example:

const validator = new Validator({
  str: new Rule(
    { type: 'string' },
    'Error message'
  )
});

validator.getErrors({
  str: undefined,
}); // Error thrown: Cannot read property 'constructor' of undefined

Expected behavior: [ 'Error message' ]

P.S. null and undefined have no constructor in JS