Required validator throws on null values

[kgkma]

Description
When a definition of type object specifies required properties, passing in null to the validate method throws a TypeError: Cannot read property 'hasOwnProperty' of null.

Steps to reproduce

const schema = {
  def: {
    type: 'object',
    properties: {
      foo: { type: 'string' }
    },
    required: ['foo'],
  },
};
const env = djv();
env.addSchema('test', schema);

const obj = null;
const errors = env.validate('test#/def', obj); // should return the errors object

Expected behaviour
The validation fails and errors is populated with the validation error.

Actual behaviour
TypeError: Cannot read property 'hasOwnProperty' of null is thrown.

Version
2.1.1