Allow empty names in import

[kousu]

Is your feature request related to a problem? Please describe.

I am often bulk importing emails using /admin/subscribers/import (to workaround #1546) of pre-existing subscribers, and I wish I didn't have to fill in a name field, especially since I'm unchecking "Overwrite".

I end up using files like

email,name,attributes
a@example.com,"name","{}"
b@hotmail.com,"name","{}"
c@gmail.com,"name","{}"
d@gmail.com,"name","{}"

but why is this a legal file

email,name,attributes
a@example.com,"name",
b@hotmail.com,"name",
c@gmail.com,"name",
d@gmail.com,"name",

but this isn't?

email,name,attributes
a@example.com,,"{}"
b@hotmail.com,,"{}"
c@gmail.com,,"{}"
d@gmail.com,,"{}"

The latter causes

skipping line 1: a@example.com: Invalid name.
skipping line 2: b@hotmail.com: Invalid name.
skipping line 3: c@gmail.com: Invalid name.
skipping line 3: d@gmail.com: Invalid name.

Describe the solution you'd like

Allow empty names during import. If that's not okay, allow it at least when "overwrite" is unchecked.

[knadh]

Please see this: #967