[Channel Manager] Make all implicit validation explicit

Functional description

The visual editor still has some implicit validations. These should be made explicit, because "Keep draft" has to be able to skip these validations. Implicit validations are validations that are imposed during updating properties but are not explicitly part of a validator.

An example of such an implicit validation is that in the channel manager a value of a multi-value property should not be null. In that case the document can still be kept as draft, but the save action should fail. Another example is a required field. That validation is implicit as well.

Approach

Write a unit test that uses the DocumentServiceImpl to store properties that don't pass the implicit validations.
Move all implicit validations to explicit ones.