Standardise appearance of missing data
Standardise appearance of missing data

This makes the rendering the same as for "created_by_name" when the
data isn't present. It's a bit more complicated for "updated_at" so
I checked that it's implicitly covered by the tests, which fail if I
remove the "if" conditional for any of these fields.

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin