Refactor Unimail Domains Import
First try at simplification in 5521d789, it seems though that on first initialisation and/or when the email domain is not part of the Unimail domains list the following error happens:
Sep 26 14:32:11 keycloak standalone.sh[9210]: 14:32:11,363 WARN [org.keycloak.services] (default task-2) KC-SERVICES0013: Failed authentication: java.lang.NullPointerException
Sep 26 14:32:11 keycloak standalone.sh[9210]: at deployment.unimail-domain-validator.jar//org.fachschaften.keycloak.form_extensions.SignupFormAction.lambda$validate$3(SignupFormAction.java:148)
Sep 26 14:32:11 keycloak standalone.sh[9210]: at java.base/java.util.stream.MatchOps$1MatchSink.accept(MatchOps.java:90)
Sep 26 14:32:11 keycloak standalone.sh[9210]: at java.base/java.util.Spliterators$ArraySpliterator.tryAdvance(Spliterators.java:958)
Sep 26 14:32:11 keycloak standalone.sh[9210]: at java.base/java.util.stream.ReferencePipeline.forEachWithCancel(ReferencePipeline.java:127)