fix possible null pointer exception and a failing Java test case

src/main/java/de/tum/in/www1/artemis/domain/Exercise.java

@@ -489,10 +489,14 @@ else if (participation.getExercise() instanceof ModelingExercise || participatio
      * @param ignoreAssessmentDueDate defines if assessment due date is ignored for the selected results
      * @return the latest relevant result in the given participation, or null, if none exist
      */
+    @Nullable
     public Result findLatestRatedResultWithCompletionDate(Participation participation, Boolean ignoreAssessmentDueDate) {
         // for most types of exercises => return latest result (all results are relevant)
         Result latestResult = null;
         // we get the results over the submissions
+        if (participation.getSubmissions() == null || participation.getSubmissions().isEmpty()) {
+            return null;
+        }
         var results = participation.getSubmissions().stream().map(Submission::getResult).filter(Objects::nonNull).collect(Collectors.toSet());
         for (Result result : results) {
             // NOTE: for the dashboard we only use rated results with completion date

src/main/java/de/tum/in/www1/artemis/domain/quiz/QuizExercise.java

@@ -6,6 +6,7 @@
 import java.util.*;
 import java.util.stream.Collectors;
 
+import javax.annotation.Nullable;
 import javax.persistence.*;
 
 import org.hibernate.Hibernate;
@@ -444,6 +445,7 @@ public StudentParticipation findRelevantParticipation(List<StudentParticipation>
     }
 
     @Override
+    @Nullable
     public Result findLatestRatedResultWithCompletionDate(Participation participation, Boolean ignoreAssessmentDueDate) {
         if (shouldFilterForStudents()) {
             // results are never relevant before quiz has ended => return null
@@ -452,6 +454,9 @@ public Result findLatestRatedResultWithCompletionDate(Participation participatio
         else {
             // only rated results are considered relevant
             Result latestRatedResult = null;
+            if (participation.getSubmissions() == null || participation.getSubmissions().isEmpty()) {
+                return null;
+            }
             // we get the results over the submissions
             var results = participation.getSubmissions().stream().map(Submission::getResult).filter(Objects::nonNull).collect(Collectors.toSet());
             for (Result result : results) {