Omit nil SMTP settings

Allows leaving `smtp_settings[:domain]` unspecified. Explicitly setting
it to `nil` (as when `OSEM_SMTP_DOMAIN` is unset) overrides the default
value of `'localhost.localdomain'` and caused SMTP connections to fail
with `EOFError`.

config/environments/production.rb

@@ -97,7 +97,7 @@ Osem::Application.configure do
     domain:               ENV['OSEM_SMTP_DOMAIN'],
     enable_starttls_auto: ENV['OSEM_SMTP_ENABLE_STARTTLS_AUTO'],
     openssl_verify_mode:  ENV['OSEM_SMTP_OPENSSL_VERIFY_MODE']
-  }
+  }.compact
 
   # Set the secret_key_base from the env, if not set by any other means
   config.secret_key_base ||= ENV["SECRET_KEY_BASE"]