[CBBS] Merge should skip merging an empty range of backups

What's the issue?
When running scheduled merge tasks, we have no validation which checks to ensure that we have two or more backups to be merged, this results in merges failing because this is being picked up by 'cbbackpumgr'.

What's the expected behavior
We should detect that the merge will fail, and skip it.

Steps to reproduce
1) Create a plan where the first task is a merge
2) Wait for the merge to trigger
3) Should see that it fails

What's the fix?
We have a couple of possible fixes:
1) We run an info beforehand, and skip running the task if there's not enough backups
2) We run the merge, and handle the error specifically

I prefer option #1 (or some variation of it) because it'll allow us to have a task status such as 'Skipped' so that users are aware that the merge has been skipped; this functionality could also be used to resolve MB-47690.