Permit null as an enum value in referenced schemas

[kylef]

The logic to check if we need to dereference a schema was failing in a case where we would find null as a value on an enum. As we're checking for the presence of null.$ref which causes an error to be thrown:

Cannot read property '$ref' of null

This is a little tricky to test as it is a combination of things happen and thus I have not created a unit regression test but only a regression integration fixture. This problem can only happen when you reference another schema, for example the following tests pass without these fixes:

it('translates x-nullable to enum value', () => {
  const schema = convertSchema({ enum: ['north', 'east', 'south', 'west'], 'x-nullable': true });
  expect(schema).to.deep.equal({ enum: ['north', 'east', 'south', 'west', null] });
});

it('translates x-nullable to enum value with existing null in enum', () => {
  const schema = convertSchema({ enum: ['north', 'east', 'south', 'west', null], 'x-nullable': true });
  expect(schema).to.deep.equal({ enum: ['north', 'east', 'south', 'west', null] });
});

It's only when convertSchema is used when the schema references another schema where you'd be able to reproduce.