In fact, the problem comes because when I get the value of the field from the form , there are 3 possible values :
1) cn=john,ou=users,o=test --> when I changed the value in the form --> ENTITY works and set the attribute value
2) empty --> when I remove a previous value --> ENTITY works and remove the attribute value
3) undefined --> when the field did not change and was already empty --> ENTITY fails as "undefined" is not a supported DN

So I followed Fernando suggestion and test field value for "undefined". If true, I do not change the attribute in entity activity , if false I can run the proper entity activity.

Works fine.

thx for help and contribution.