Added null value for list.

No ValidationError raised, If  "many=True" and "required=True" are set on PrimaryKerRelatedField.

````python
class CompanySerializer(serializers.ModelSerializer):
    groups = serializer.PrimaryKeyRelatedField(many=True, required=True)
    class Meta:
        model = Company
````	

These PUT requests should raise a ValidationError as no groups defined.
````
{"name":"Company Name Ltd.","groups":[]}
{"name":"Company Name Ltd."}
````

Expected error: 
````
{"groups": ["This field is required."]}
````