Make sure we don't have nulls lying around

src/main/java/org/fachschaften/keycloak/form_extensions/SignupFormAction.java

@@ -180,7 +180,12 @@ public class SignupFormAction implements FormAction, FormActionFactory {
 
         Stream<String> unimailDomainsStream = Arrays.stream(unimailDomains);
 
-        if (!unimailDomainsStream.anyMatch( f -> email.endsWith(f) )) {
+        if (
+            // If `email` is null or empty it's not an unimail address
+            email == null || email.trim().length() == 0 ||
+            // If `email` doesn't end with at least one unimail domain
+            unimailDomainsStream.noneMatch( f -> f != null && email.endsWith(f) )
+        ) {
             errors.add(new FormMessage(Validation.FIELD_EMAIL, "notInUnimailDomainsMessage"));
         }