gossip: Remove empty string feature in get_supported_features
gossip: Remove empty string feature in get_supported_features

If the feature string is empty, boost::split will return
std::set<sstring> = {""} instead of std::set<sstring> = {}
which will make a node with a feaure, e.g. std::set<sstring> =
{"RANGE_TOMBSTONES"}, think it does not understand the feature of
a node with no features at all.

avikivity/scylla,kjniemi/scylla,kjniemi/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,raphaelsc/scylla,scylladb/scylla,duarten/scylla,kjniemi/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,raphaelsc/scylla,avikivity/scylla,raphaelsc/scylla