Support editable 3p accounts without nicknames

Fix a theoretical crash case.

We shouldn't assume the nickname field always exists.
All standard writable accounts have nickname fields.
But, it is possible to create an 3p account type with
an xml edit specification that omits the nickname
(I don't know if anyone does this).

Bug: 18004959
Change-Id: I9770e88891e5800ef56d2df8a0ddb8463e3e8815