[CBBS] Panic whilst cleaning up after failure archive creation

What's the issue?
We have some cleanup which is run after failing to create an active repository; this cleanup will handle removing newly created credentials (for cloud/encryption).

We always run this cleanup (which deletes a path in KV), the underlying library has validation which asserts that a path doesn't end in '/'.

This, matched with the fact that the newly created credentials name can be empty, means we should fail this assertion.

What's the fix?
Only delete the credentials if we actually created them.