Mandrill Transactional is trying to send update email although no recipient set

Problem/Motivation

When you have the Update module enabled and there are available updates but you have no email set in the "Email addresses to notify when updates are available" field (description says "If blank, no emails will be sent.") and then you run drush cron you'll get the following error:

 [error]  Client error: `POST https://mandrillapp.com/api/1.0/messages/send-template` resulted in a `400 Bad Request` response:
{"status":"error","code":-2,"name":"ValidationError","message":"Validation error: {\"message\":{\"to\":[{\"email\":\"Sor (truncated...)

 [error]  Failed sending email from mail@heicosportiv.de to recipient. error: Client error: `POST https://mandrillapp.com/api/1.0/messages/send-template` resulted in a `400 Bad Request` response:
{"status":"error","code":-2,"name":"ValidationError","message":"Validation error: {\"message\":{\"to\":[{\"email\":\"Sor (truncated...)

 [error]  Error sending email (from mail@heicosportiv.de to  with reply-to not set).
 [error]  Message: Mailchimp Transactional: /Client error: `POST
https://mandrillapp.com/api/1.0/messages/send-template` resulted in a `400
Bad Request` response:
{"status":"error","code":-2,"name":"ValidationError","message":"Validation
error: {\"message\":{\"to\":[{\"email\":\"Sor (truncated...)
/

 [error]  Message: Unable to send email. Contact the site administrator if the problem persists.

Steps to reproduce

This only works when there are available updates:

1. Mailchimp Transactional correctly configured and test mail sent successfully ✅
2. Enable Update module
3. Go to /admin/reports/updates/settings
4. In textarea "Email addresses to notify when updates are available" put no email address and save
5. drush cron

6.  [error]  Client error: `POST https://mandrillapp.com/api/1.0/messages/send-template` resulted in a `400 Bad Request` response:
   {"status":"error","code":-2,"name":"ValidationError","message":"Validation error: {\"message\":{\"to\":[{\"email\":\"Sor (truncated...)
   
    [error]  Failed sending email from mail@heicosportiv.de to recipient. error: Client error: `POST https://mandrillapp.com/api/1.0/messages/send-template` resulted in a `400 Bad Request` response:
   {"status":"error","code":-2,"name":"ValidationError","message":"Validation error: {\"message\":{\"to\":[{\"email\":\"Sor (truncated...)
   
    [error]  Error sending email (from mail@heicosportiv.de to  with reply-to not set).
    [error]  Message: Mailchimp Transactional: /Client error: `POST
   https://mandrillapp.com/api/1.0/messages/send-template` resulted in a `400
   Bad Request` response:
   {"status":"error","code":-2,"name":"ValidationError","message":"Validation
   error: {\"message\":{\"to\":[{\"email\":\"Sor (truncated...)
   /
   
    [error]  Message: Unable to send email. Contact the site administrator if the problem persists.

Proposed resolution

Not sure where the actual fix should be implemented exactly. I would naively think that the trigger should never have reached mailchimp_transactional in the first place.

But when I check for an empty to-address inside mailchimp_transactional/modules/mailchimp_transactional_template/src/TemplateService.php I can at least circumvent that error.

Remaining tasks

User interface changes

API changes

Data model changes