Ignore attributes with null value

Problem/Motivation

We received example articles with attributes that had null as value instead of true. Documentation is not very clear and only explains it in the context of the retain operation that we don't use/support: https://github.com/ottypes/rich-text#retain-operation but I think it is safe to skip any attribute definition if the value is NULL and not true or something else.

Steps to reproduce

Extend test with a bold: null attribute, ensure that the bold template is not used for it.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes