refactor tests after merging master

Co-Authored-By: tomvahl <tomvahl@users.noreply.github.com>
Co-Authored-By: kasch309 <kasch309@users.noreply.github.com>

src/main/java/mops/gruppen2/service/ControllerService.java

@@ -133,6 +133,7 @@ public class ControllerService {
     }
 
     private List<User> readCsvFile(MultipartFile file) throws EventException, IOException {
+        if(file == null) return new ArrayList<>();
         if (!file.isEmpty()) {
             try {
                 List<User> userList = CsvService.read(file.getInputStream());