As far as I can tell, there are no properties with invalid datatype in the query service (http://tinyurl.com/kvabzcp), but I don’t know what the exporter or the updater would do with such properties. Perhaps Quarry would be better suited for this?

Edit: Nothing found on Quarry either (https://quarry.wmflabs.org/query/18830).

According to wb_property_info, no such broken properties with an empty property type exist on wikidata:

MariaDB [wikidatawiki_p]> select * from wb_property_info where pi_type = "";
Empty set (0.00 sec)

Edit: oh, just saw the quarry link. Never mind :)

As to solving this issue: PropertyContent::isValid should at least check that the datatype is not empty. PropertyHandler::createEmptyEntity should probably fail.

EntitySavingHelper::isEntityCreationSupported needs to distinguish between entities that can be created without additional information, and entities that do need additional information. We already distinguish whether an explicit ID is needed (or allowed) for creation. This mechanism would need to be extended.

Note that we introduced the ability to create entities from all the edit modules on purpose, to support automatic materialization of "virtual" MediaInfo entities. If we remove that ability, the UI for MediaInfo will need quite a bit of work.

Change 355443 had a related patch set uploaded (by Daniel Kinzler; owner: Daniel Kinzler):
[mediawiki/extensions/Wikibase@master] Prevent creation of Properties without data type [WIP].

https://gerrit.wikimedia.org/r/355443

Btw, I suspect that other API modules that use EntitySavingHelper are also affected, e.g. SetClaimValue, RemoveQualifiers, etc. Basically, all API modules that modify an entity.

Change 355443 merged by jenkins-bot:
[mediawiki/extensions/Wikibase@master] Prevent saving of Properties with an empty datatype.

https://gerrit.wikimedia.org/r/355443