Improve null handling in XML serializer

[sobolewskikamil]

Problem description
During the course of JSON serializer implementation, we have understood there are serialization cases which are not handled properly in XML serialization logic. Null values in both Rule model and XML might not be handled and eventually result in NullPointerException being thrown. Those cases should be recognized and covered.
TIP:
Refer to JSON serializer code and null case tests.

Suggested solution

• Recognize not covered null cases in XML serializer
• Fix them and cover those changes with tests

Acceptance criteria

• Tests are prepared
• Code review is done