The Release importer should handle null values for annotations

[Nicklas Nordborg]

The issue we need to solve is a case where INCA annotations have had a value in one release but it has been removed in an update. In the current situation it means that the original value may exists on Relax as a default value and it will be used also for the new release.

This change also need a corresponding change in the Release exporter in the Reggie plugin. See ticket #1398. When a null value is present in the JSON, the importer may need to take some actions:

• If the current item has a default value for the annotation, the default value should be removed.
• The default value should be copied to project-specific annotations. One for each release that the item (and annotation type) is part of.
• If the item doesn't have a default value, nothing should be done.
• Already existing project-specific values should not be modified.

For example:

• Item A is part of release 1.0, 1.1 and 2.0.
• It has a default annotation value: foo=bar
• In release 3.0 the value has been removed.
• The default value is removed and three new project-specific values for 1.0, 1.1 and 2.0 is created: foo=bar.

• Item B is part of release 1.0, 1.1 and 2.0
• It has a default value: foo=bar and a project-specific values for 1.1 and 2.0: foo=bar2.
• In release 3.0 the value has been removed.
• The default value is removed and one new project-specific value for 1.0 is created: foo=bar. The project-specific values for 1.1 and 2.0 are not modified.