Fix Protocol.UndefinedError

[neominik]

Exq.Redis.JobStat.node_ping/3 returns nil when Redis returns an unexpected response, but Exq.Node.Server.process_signals/2 doesn't handle nil and crashes when calling Enum.each(nil, ...).

[coveralls]

coverage: 91.456% (+0.07%) from 91.382%
when pulling 3c0ee2f on neominik:patch-1
into 5f31c10 on akira:master.