Minor usnic fixes

Two commits:

1. Do not send zero-length modex messages if there are no usnic devices.  On master, open-mpi/ompi@ec4268b59cf42749b3975acc75be013e31925cf8
1. Do not error if a peer did not supply a usnic modex message -- it just means that there are no usNIC BTL modules in that peer.  This has actually be on master/trunk for forever; I just noticed today that it was not on the v1.8 branch.

Reviewed by @goodell