Should deliver and deliver_async noop if all recipients are blank?

[paulcsmith]

While using this in one of our applications we had to do something like this

users = Repo.all(User)

if length(users) > 0 do
  Emails.welcome_email(users) |> Mailer.deliver_async
end

Maybe deliver_async should throw a warning? Maybe an option to allow empty_recipients?

deliver_async(allow_empty_recipients: true)

Maybe this could be done at the mailer level via an option?

defmodule MyApp.Mailer do
  use Bamboo.Mailer, otp_app: :my_app

  @allow_empty_recipients true
end

[paulcsmith]

Maybe it should raise only if the to, bcc or cc is nil since by default those are empty lists.

If you do something like a wrote above it would still be an empty list, whereas setting it to nil probably means there was a problem where you expected the recipient to be something and it was accidentally nil.

[paulcsmith]

Decided on: #40 (comment)