Fix ezp 27254 unknow relation type 0

[pkamps]

The issue:
https://jira.ez.no/browse/EZP-27254

I pull request that got merged to fix the issue:
ezsystems#1295

This pull request is different to the upstream pull request. This pull request fixes the root cause and is not fixing the symptom. See my comment on June 27 ezsystems#1295

How to test this pull request:

1. Create a new article (or another content class with 2 ezxmltext attributes)
2. Embed objects in 2 ezxmltext attributes (for example "Summary" and "Body")
3. Send for publishing
4. Check the DB content with: "select * from ezcontentobject_link where from_contentobject_id = ;"
5. Confirm you see 2 entries for version 1
6. Re-publish the article
7. Confirm you see 2 entries for version 2

Try other scenarios with multiple languages, and also removal of embedded objects in the ezxmltext attributes.