Saving SMTP Transport deletes existing password

Thanks for the issue. In fact two issues😃.

1) is a bug, bit irritating to users, fairly quick to fix, hopefully soon.
2) is a new feature, quite complex and needs some thought, might take some time. swiftmailer used the key module directly, and the credential providers were hard-coded (not plugins). We should make a solution that will extend to all the 3rd-party transports.

Please can you raise a separate issue for 2)?

@Jons, I do something like this in one of the modules I maintain. Leaving the password field blank doesn't overwrite the existing one. There's a separate checkbox for deleting the stored password.

In submitForm():

// Only save the password if it is not empty.
if (!empty($values['password'])) {
  $config->set('password', $values['password']);
}

// Check option to delete the password.
if (!empty($values['delete_password'])) {
  $config->set('password', '');
}

Similar to #5, a slightly different way.