`all_objects` method is returning empty objects

[sneridagh]

The generator method in:

https://github.com/plone/plone.exportimport/blob/main/src/plone/exportimport/exporters/content.py#L28-L48

was just returned me None elements, no clue why, I had to put a try in the caller to make it work. Could be that something was wrong in my DB, nevertheless, maybe it's safer to add it anyways.