skip nil and zero length hits, fix #556

[NeoCN]

Focus on the flowing multiread function code snippet

	var traces []*model.Trace
	for _, result := range results.Responses {
		if result.Hits == nil {
			return nil, errNilHits
		}
		spans, err := s.collectSpans(result.Hits.Hits)
		if err != nil {
			return nil, err
		}
		traces = append(traces, &model.Trace{Spans: spans})
	}
	return traces, nil

when trace id is invalid or not in the es store, we got response json as follows:

{
   "responses":[
      {
         "took":2,
         "timed_out":false,
         "_shards":{
            "total":15,
            "successful":15,
            "failed":0
         },
         "hits":{
            "total":0,
            "max_score":null,
            "hits":[]
         },
         "status":200
      }
   ]
}

and then the response json is unmarshalled to the elastic.MultiSearchResult struct, you will notice that the snippet

"hits":{
    "total":0,
    "max_score":null,
    "hits":[]
},

will not get nil result.Hits, but a zero length result.Hits.Hits.

When len(result.Hits.Hits) == 0, collectSpans will return []*model.Span with zero length and no error.

In this case, got result traces as

[]*model.Trace{
    &model.Trace{
        Spans: {
        },
        Warnings: nil,
    },
}

this results that len(traces) == 0 not triggered, and then no errNoTraces is returned.

As explained above, neither nil result.Hits nor zero length result.Hits.Hits should do the following collectSpans, just skip them.

[coveralls]

Coverage remained the same at 100.0% when pulling 0ba832d on NeoCN:master into 05c9958 on jaegertracing:master.

[yurishkuro]

Thanks!

Fixes #556