Allow device factory to return nil for unrecognized nodes.
Allow device factory to return nil for unrecognized nodes.

Signed-off-by: Jon Seymour <44f878afe53efc66b76772bd845eb65944ed8232@ninjablocks.com>

ninjasphere/go-openzwave,ninjasphere/go-openzwave,ninjasphere/go-openzwave,ninjasphere/go-openzwave,ninjasphere/go-openzwave,ninjasphere/go-openzwave