KCountrySubdivision: unbreak support of iso-codes >= 4.16
iso-codes 4.16 changed the data schema, by the following commit not giving any info what is fixed there exactly: "Fix: always use country code for parent in ISO 3166-2"
Patch is first rough approach, assuming that the graceful handling of both cases might not be that expensive on the initial cache creation.
Other option might be to use code conditional on found iso-codes version. Though as this is a run-time version, we might need to adapt to the package being changed/upgraded behind our back from the old to the new.
Needs backport to KF5