get bandwidth limit: improve detecting if storages are involved

get bandwidth limit: improve detecting if storages are involved

Previously, calling with e.g. $storage_list = [undef] would lead to an
early return of $override and not consider the limit from
datacenter.cfg.

Refactoring the bandwidth limit handling for migration introduced
calls such as described above, which broke applying the limit from
datacenter.cfg for VM RAM/state migration.

Reported in the community forum:
https://forum.proxmox.com/threads/37920/post-513005

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>