Lookup index, rather than iterating
Lookup index, rather than iterating

Since instances of `Recipients` are now iterable we can just look up the
first row by index, rather than having to hit the rows property as a
generator.

There is a chance that there won’t be a first row, if the file has 1 or
fewer columns. So we still need to handle a possible exception (albeit
it a different one to before).

alphagov/notifications-admin,gov-cjwaszczuk/notifications-admin,gov-cjwaszczuk/notifications-admin,gov-cjwaszczuk/notifications-admin,gov-cjwaszczuk/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin