Fix nil approvals_before_merge migrated as 0 approvals_required

What does this MR do?

When setting MR's approvals_before_merge to blank value, it would actually set it to nil. In the current behavior, the user would see the project's approvals_before_merge instead.

This MR simulate this behavior, so if nil is set, we set ApprovalMergeRequestRule's approvals_required with project's setting.

What are the relevant issue numbers?

#8647 (closed)

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• Security reports checked/validated by reviewer