usnic: do not send zero-length modex message

If there are no usnic BTL modules, then just avoid sending any modex
message at all (other BTLs do this; it's safe to do).

The change is smaller than it looks: I added a "if 0 ==..." check at
the top to return immediately if there are no BTL modules.  Then I
removed some now-unnecessary conditionals and un-indented as
appropriate.

Fixes #248