Resolve "[Queue-2.0] Unenrolling from an edition causes blocked status"

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Closes #438 (closed)

src/main/java/nl/tudelft/queue/cache/StudentGroupCacheManager.java

@@ -26,6 +26,7 @@ import nl.tudelft.labracore.api.dto.StudentGroupDetailsDTO;
 import nl.tudelft.labracore.api.dto.StudentGroupSummaryDTO;
 
 import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.lang.Nullable;
 import org.springframework.stereotype.Component;
 import org.springframework.web.context.annotation.RequestScope;
 
@@ -60,7 +61,10 @@ public class StudentGroupCacheManager extends CoreCacheManager<Long, StudentGrou
 	}
 
 	@Override
-	protected List<StudentGroupDetailsDTO> fetch(List<Long> ids) {
+	protected List<StudentGroupDetailsDTO> fetch(@Nullable List<Long> ids) {
+		if (ids == null || ids.isEmpty()) {
+			return List.of();
+		}
 		return api.getStudentGroupsById(ids).collectList().block();
 	}