Handle external issues in IssueReferenceFilter

What does this MR do?

This MR fixes handling of external issues in IssueReferenceFilter.

Are there points in the code the reviewer needs to double check?

No.

Why was this MR needed?

Handling of external issues was broken when I refactored IssueReferenceFilter to use fewer SQL queries.

What are the relevant issue numbers?

#18827 (closed)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added Not really needed since the changes this fixes weren't released in a previous minor release
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together