[views] csvexport accept an empty rset (closes #4236928) When you tried to apply the 'csvexport' view on an empty rset, the view couldn't be selected and you got a HTTP 500 error. Also add two new test cases.

[views] Display attributes in entity creation form based on "add" permission Previously, the "update" permission was used. Hence in case the latter is more restrictive that the "add" permission, an user may not be able to set such an attribute, despite she may have "add" permission on it. As a result of the change of permissions action in `editable_attributes` method (add/update depending on whether the entity is being created or modified), the "eid" attribute would have shown up in the edition form. To avoid this, it is moved in the "hidden" section (where it should arguably belong anyways). Closes #4342844.