Error after changing an article without setting a Changer

[thomasduenser]

Q	A
Bug?	yes
New Feature?	no
Bundle Version	dev-develop

Actual Behavior

When saving an article - after changing it via e.g. a Command without setting a changer - following error is thrown:

Entity with the type "" and the id "null" not found.

Request URL:http://oelz.dev/admin/api/users/null
Request Method:GET
Status Code:404 Not Found

Expected Behavior

It should not throw an error.

Steps to Reproduce

1. Create an article and save it
2. Update / change the article via a command without setting a changer
3. Open the article in the backend and save.

Possible Solutions

Do not try to fetch a user when the changer is null or use the creator as changer.