prevent statements from being created if a *_item field is not a Q-value

[mhl]

This creates difficult to diagnose problems, e.g. the error:

Error from the Wikidata API [invalid-snak] Invalid snak data.

... could be caused if parliamentary_group_item is something other than a Q-value.

[tmtmtmtm]

NB this does show up in the non-JS version, as we're presumably running things through the {{Q|…}} template there. But once the JS version has loaded that disappears/gets hidden, so is much easier to overlook.