Empty ISSN keys sometimes produced

[osma]

A work key "issn:" is produced e.g. for this record:

000254001 4901  L $$aPoliisiosaston julkaisu / Sisäasiainministeriö,$$x;$$v7/2001
000254001 830 0 L $$aPoliisiosaston julkaisu / Sisäasiainministeriö,$$x;$$v7/2001.

The problem is the empty $x subfield, but we could detect and avoid these.