This function may be called with @dconnuri == NULL, e.g. from
virDomainMigrateToURI3() if the flags are missing
VIR_MIGRATE_PEER2PEER flag. Moreover, all later functions called
from here do wrap it into NULLSTR() so why not do the same here?
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>