In ContentStaging, the sync will fail if a select with value zero and required attribute is set.
Steps to reproduce
1) Create a content class with an attribute selection set as required.
2) In the selection attribute, define two values: 0 and 1.
3) Create a content object, leave the attribute in default value: 0
4) Try to sync
Can't you just push this to your fork, and send the PR from there ?
Fixed in master: https://github.com/ezsystems/ezpublish-legacy/commit/02d28526d2d702760f0f6c2377c5b4a2d7f8ca39