UHF-9514: Support users without email

[tuutti]

user_load_by_mail() dies to a InvalidQueryException if the user's email address is not set:

helfi-etusivu-app  | NOTICE: PHP message: Uncaught PHP Exception Drupal\Core\Database\InvalidQueryException: "Query condition 'users_field_data.mail IN ()' cannot be empty." at /app/public/core/lib/Drupal/Core/Database/Query/Condition.php line 106
helfi-etusivu-app  | 2024/08/05 07:26:28 [error] 50#50: *743 FastCGI sent in stderr: "PHP message: Uncaught PHP Exception Drupal\Core\Database\InvalidQueryException: "Query condition 'users_field_data.mail IN ()' cannot be empty." at /app/public/core/lib/Drupal/Core/Database/Query/Condition.php line 106" while reading response header from upstream, client: 172.29.0.2, server: _, request: "GET /en/user/193/edit HTTP/1.1", upstream: "fastcgi://127.0.0.1:9000", host: "helfi-etusivu.docker.so", referrer: "https://helfi-etusivu.docker.so/en/user/193"