virDomainMigrateUnmanagedParams: Don't blindly dereference @dconnuri
    
    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: Michal Privoznik <mprivozn@redhat.com>