Add support for null_flavour at the ELEMENT level

[ppazos]

Background

In the FHIR bridge we could have missing data, or even data that is semantically similar to the openEHR ELEMENT.null_flavour, but currently the code generated doesn't allow to record the null_flavour.

Acceptance criteria

At the same level the fields for ELEMENT.value are generated, we need to generate field for the ELEMENT.null_flavour which is a DV_CODED_TEXT.

Success criteria

• Review / Merge request approved (P2P session)
• Unit tests passed
• Updated documentation (Javadoc and Sphinx)
• Acceptance criteria fulfilled
• Build without errors
• Release notes prepared
• Runtime warnings

[stefanspiska]

Fixed in #153 which will become release 1.0.0
see
https://github.com/ehrbase/openEHR_SDK/blob/release/v1.0.0/CHANGELOG.md for changes
https://github.com/ehrbase/openEHR_SDK/blob/release/v1.0.0/UPDATING.md for how to update