Fix logic for email address text field

* Update the state of the button for every change to the
email in the textfield so it is disabled as soon as the
email is not valid or new anymore.
* Make empty string the default value of the textfield email
instead of undefined if serviceuser has no email set.
* Make empty string a valid email if serviceuser already
has an email set. To make it possible to remove email from
serviceuser.

Change-Id: I1cc5ae58d97db64af58ac95e895cffe11efe967d