Follow-up from "Allow approvals_required to be lower than project"

The following discussion from !14902 (merged) should be addressed:

• @pslaughter started a discussion: (+2 comments)

  issue: There's a small issue when no approvers are added at the project level.

  20190805_fallback_rule_override

  The BE is automatically flooring the MR approvals required to the project level approvals required. This is because the "fallback rule" validation is handled differently than the regular rule :/

  We could try to take care of this here or in a follow up MR. What do you think?

When there are no project-level approval rules defined we have a fallback rule in place. If Can override approvers and approvals required per merge request is enabled, MR author can override the number of approvals required for that fallback rule.

Currently, author can set it (approvals_before_merge) to be greater than or equal to the value set at the project level. If set to a lower value, no error is shown and gets reset to the project level value.