Fix empty request table error

src/main/java/nl/tudelft/queue/service/RequestTableService.java

@@ -347,6 +347,9 @@ public class RequestTableService {
 	 * @return             The mapping from assignment ids to edition names.
 	 */
 	private Map<Long, String> assignmentsWithCourseCodes(List<AssignmentDetailsDTO> assignments) {
+		if (assignments.isEmpty())
+			return Collections.emptyMap();
+
 		List<Long> assignmentIds = assignments.stream().map(AssignmentDetailsDTO::getId).toList();
 
 		var assignmentModules = Objects