Don't print the pool option name if it's null.
Before:
virsh # vol-name vol
error: failed to get vol 'vol', specifying --(null) might help
error: Storage volume not found: no storage vol with matching path vol
After:
virsh # vol-name vol
error: failed to get vol 'vol'
error: Storage volume not found: no storage vol with matching path vol
Bug: https://bugzilla.redhat.com/show_bug.cgi?id=924571