fix(service): add @Nullable to findOrCreateByAlias and filter nulls in caller

Add @Nullable annotation to findOrCreateByAlias() return type.
Filter null results (from SKIP classification) in MassImportService
receiver list to prevent null elements in the receivers collection.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/service/MassImportService.java

@@ -3,6 +3,7 @@ package org.raddatz.familienarchiv.service;
 import lombok.RequiredArgsConstructor;
 import lombok.extern.slf4j.Slf4j;
 import org.apache.poi.ss.usermodel.*;
+import java.util.Objects;
 import org.raddatz.familienarchiv.exception.DomainException;
 import org.raddatz.familienarchiv.exception.ErrorCode;
 import org.raddatz.familienarchiv.model.Document;
@@ -301,6 +302,7 @@ public class MassImportService {
         Person sender = senderRaw.isBlank() ? null : findOrCreatePerson(senderRaw);
         List<Person> receivers = PersonNameParser.parseReceivers(receiversRaw).stream()
                 .map(this::findOrCreatePerson)
+                .filter(Objects::nonNull)
                 .toList();
 
         Tag tag = null;

backend/src/main/java/org/raddatz/familienarchiv/service/PersonService.java

@@ -1,9 +1,12 @@
 package org.raddatz.familienarchiv.service;
 
 import java.util.List;
+import java.util.Objects;
 import java.util.Optional;
 import java.util.UUID;
 
+import org.springframework.lang.Nullable;
+
 import org.raddatz.familienarchiv.dto.PersonNameAliasDTO;
 import org.raddatz.familienarchiv.dto.PersonSummaryDTO;
 import org.raddatz.familienarchiv.dto.PersonUpdateDTO;
@@ -59,6 +62,7 @@ public class PersonService {
         return personRepository.findByFirstNameIgnoreCaseAndLastNameIgnoreCase(firstName, lastName);
     }
 
+    @Nullable
     @Transactional
     public Person findOrCreateByAlias(String rawName) {
         String alias = rawName.trim();