usnic should not send an empty modex when there are no usNIC devices

[jsquyres]

Per #235, @ggouaillardet says that the usNIC BTL is sending around an empty modex when there are no usNIC devices (in v1.8).

Ralph says that this may be necessary in v1.8; need to investigate and see if usnic actually needs to do this. If not, usnic should not send around empty modex messages.

On master, a quick look indicates that usnic is probably not sending an empty modex around, but that should be double checked.

[jsquyres]

Confirmed: if the probe for usnic devices succeeds but turns up no devices, we'll send an empty modex message around (both master and v1.8). Doh!