goarista/gnmi: correctly (don't) set extentions if none supplied
goarista/gnmi: correctly (don't) set extentions if none supplied

my earlier testing was clearly focused on the new -arbitration
flag; trying to do a normal set got broken - correcting now to
not try and send a nil extension in the set request.

Change-Id: If411a79479750acb3cc3e324bef42c553f148bf6

aristanetworks/goarista,aristanetworks/goarista