Token replacement doesn't work when empty

As far as I can see this has been like this since the start of the D8 branch. The code is in MailEntity::getBodyWithFormat(). Passing the 'clear' option to token replace would fix it to work how you want.

Great -- and where/how do I pass a "clear" option?

In $options when calling \Drupal::token()->replace().

It's strange - I would expect [entity:field_xxx] to return blank if the field was empty and that it would only get left in when the field didn't even exist on that entity.

There's an issue for that in the token module.

Thanks @Berdir - I guess it's this issue: #2648180: Replace field tokens by empty strings when there is no field value →

So maybe we close this one as "working as designed" and anyone interested can put a patch for the token module fix?

Well, seems the token field issue has't been resolved in years, so I'm not going to give that a try. I rather write module which puts all the filled in fields from the subscription form in the right order to make for a single valid greeting field … Thanks for the heads up anyway, maybe I get good enough with coding to tackle that issue in the token module - but please don't hold your breath.