Display epic issues with null relative position

What does this MR do?

Epic issues with null relative position were not being shown on epic children widget.

This was happening because of BatchEpicIssuesLoader was fetching issues by relative position which could be null and filtered out.

I did not find a way to extend EachBatch to be able to query batches even if nil values are present, so we decided to use a boring solution. For more information check #207898 (comment 326282857).

related to #207898 (closed)