fix #10: Prevent showing "Reference not found" when refusing to select any reference

[xUser5000]

This is a fix to issue #10

What has been done

• Make showCitationPopup() return an empty string when there is no selected reference (previously it was returning undefined when encountering such situation).
• Check if (referenceId === "") return; in add-bibtex-reference.command.ts

[laurent22]

Ok that makes sense.