Display reverts to email if not set

contact edit form
if no first name, middle name, last name is set, on save, the display name generation does not take place and whatever was the previous value is retained. the display name should also be generated if an email exists (since a contact record may be defined by only an email) and should revert to the email if no computed value is generated.

the scenario exists if a contact record has a value in one of the name fields, which are then edited and removed. the record should construct the display name as the email address. instead it retains the previous value