Fix bug: empty subscription is erroneously added
Fix bug: empty subscription is erroneously added

If the client config JSON doesn't contain any subscriptions and
SENSU_CLIENT_SUBSCRIPTIONS is not set (or is empty), then
an empty subscription is added, which causes the client to enter
an endless loop of errors:

[E 161122 23:29:05 rabbitmq.go:104] Can't declare the exchange:
    Exception (504) Reason: "channel/connection is not open"

upfluence/sensu-client-go,upfluence/sensu-client-go