[aws] Don't index empty AWS Security Hub responses (#)

Set `response.split.ignore_empty_value: true` for splitting responses
from AWS Security Hub, to avoid indexing the empty wrapper document
when the Insights or Findings list is empty.

The [`response.split` documentation][1] says:

> If the split target is empty the parent document will be kept.
> If documents with empty splits should be dropped, the
> `ignore_empty_value` option should be set to `true`.
 
 [1]: https://www.elastic.co/guide/en/beats/filebeat/current/filebeat-input-httpjson.html#response-split