Subscriber::loadByMail(): Argument #1 ($mail) must be of type string, null given

Seeing more cases in tests, will update later.

This fixes some more cases.

Thanks. We also have #3031919: Bugs if user has blank email address → which is perhaps a duplicate. It would be great to have some tests.

Patch moved to #3031919: Bugs if user has blank email address → , still needs work for tests and some of the categories.

danielwolf → changed the visibility of the branch 3402475-subscriberloadbymail-argument-1 to hidden.