Maintenance Policy: .condition.operations should NOT have fewer than 1 items

Since implementation of conditions in maintenance mode THREESCALE-6552 some condition is required by UI, the policy can not be updated without that.

This is the behavior of UI only, adding the policy via API works without conditions.

This is kind of regression as UI doesn't allow maintenance mode without definition of extra condition anymore.

Reproducer: Use UI, add product/backend, go to Product->Integration->Policies, click "Add policy", add "Maintenance Mode", click "Update Policy", here it stops. Alternatively click "Red X" to delete "the operation form" and click "Update Policy" to be stopped again.

In previous version steps: click "Add policy", add "Maintenance Mode", click "Update Policy" were working.

Workaround: Add always passing condition e.g. "1 == 1"