Empty state for branch specific MR approvals

For now we don't have the minimum required approval rule in MR approvals for a specific branch. We have a spike to find out if it's possible - #480323 (closed)

In the meantime, when viewing branch rule details for a branch that doesn't have any MR approval rule set, the table renders with just a header and no cells. We could add a row with some sort of empty state message.