Optional enum fields in type structs don't handle null values properly

[psstoev]

When an enum field in struct is made {required: false} and the value for it is set to null the null stays even if there is a default value. If there isn't a default value the null replaces undefined and the enum value is later written as null in the JSON (instead of missing from the result).
Example test:

it.('allows complex type definition (optional non-default enums)', function() {
	api.enum('TestEnum', {A: 1, B: 2});
	api.type('Test', {
		notRequiredDefault: {
			type: 'TestEnum',
			required: false,
			default: 1,
		},
		notRequiredNoDefault: {
			type: 'TestEnum',
			required: false
		}
	});

	const testType = api.type('Test');

	expect(testType.convert({
		notRequiredDefault: null,
		notRequiredNoDefault: null
	})).to.deep.eq({
		notRequiredDefault: 1,
		notRequiredNoDefault: undefined
	});
});

and the failure:

AssertionError: expected { Object (notRequiredDefault, notRequiredNoDefault) } to deeply equal { Object (notRequiredDefault, notRequiredNoDefault) }
+ expected - actual

{
-  "notRequiredDefault": [null]
-  "notRequiredNoDefault": [null]
+  "notRequiredDefault": 1
+  "notRequiredNoDefault": [undefined]
}