Attributes Cannot Be Deleted Via Delete Button

[SzalonyNiemiec]

When pressing the delete button of an attribute in the edit entity view, the attribute disappears in the view, but this change is not saved. When pressing "update" and then inspectint the entity, the deleted attribute will still be visible.

Steps to reproduce:

1. Create an entity with attributes
2. Edit the entity, and press the delete button of an attribute
3. press update
4. Look at the entity again

[djs0109]

This issue is fix in the current development branch. However, the last attribute can not be deleted. The reason is probably, that we are not allowing empty attributes during the validation