Previously, the BSL credential field would always be set when using the
`create` command, even if no credential details were provided. This
would result in an empty `SecretKeySelector` in the BSL which would
cause operations using this BSL to fail as Velero would attempt to fetch
a `Secret` with an empty name from the K8s API server.
With this change, the `Credential` field is only set if credential
details have been specified. This change also includes some refactoring
to allow the change to be tested.
Signed-off-by: Bridget McErlean <bmcerlean@vmware.com>