Fixing multiEventFetch error reporting when no events are returned

[vjsamuel]

When a metricbeat module implements func (m *MetricSet) Fetch() ([]common.MapStr, error) and returns an error, it is not added to the output event because of:

events, err := fetcher.Fetch()
for _, event := range events {
	reporter.ErrorWith(err, event)
}

when events is of size 0 this is never executed and this prevents updating of module metrics like:

  "metricbeat": {
    "prometheus": {
      "collector": {
        "events": 0,
        "failures": 0,
        "success": 0
      }
    }
  },

which can be very misleading.

This PR fixes the same by checking size of the event list and passing a nil event and the error object.

[elasticmachine]

Jenkins standing by to test this. If you aren't a maintainer, you can ignore this comment. Someone with commit access, please review this and clear it for Jenkins to run.

[elasticmachine]

Jenkins standing by to test this. If you aren't a maintainer, you can ignore this comment. Someone with commit access, please review this and clear it for Jenkins to run.

[ruflin]

jenkins, test it

[ruflin]

Looks like a regression we introduced when adding the new interfaces. Thanks for fixing.