Fix related work item links syncing

What does this MR do and why?

Do not call Epics::RelatedEpicLinks::CreateService when target work items don't have a synced epic. If we call this service with the param target_issuable: [] it raises an error in IssuableLinksCreateService#render_not_found_error? which rollback the transaction.

MR acceptance checklist

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Before	After

How to set up and validate locally

1. Create a work item epic and a task
2. Visit 127.0.0.1:3000/-/graphql-explorer and use the following mutation to link the task to the work item epic

mutation { 
    workItemAddLinkedItems(input: {id: "gid://gitlab/WorkItem/WI_EPIC_ID", workItemsIds: ["gid://gitlab/WorkItem/TASK_ID"]}) {
    errors
    workItem {
      title
      widgets {
        ... on WorkItemWidgetLinkedItems {
          type
          linkedItems {
            edges {
              node {
                workItem {
                  title
                }
              }
            }
          }
        }
      }
    }
  }
}

3. Verify that no error is present.