Fix approvals before merge error while importing projects

Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/2617

This was also a customer issue: https://gitlab.zendesk.com/agent/tickets/81403

This normally happens when:

1. Approvals before merge set to 1 on the original project
2. MRs are open and closed with that 1 approval
3. Approvals before merge are then for instance set to zero (on the original project)
4. Project gets exported with that value, but validation fails importing the MR with the old value

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?