Resolve "Reviewing Badges: Zusätzliche Icons"

Related to #397 (closed)

src/main/java/at/ac/uibk/gitsearch/es/model/ArtemisExerciseInfo.java

@@ -106,6 +106,8 @@ public class ArtemisExerciseInfo extends ExerciseInfo {
         exerciseInfo.setDescription(this.getDescription());
         if (this.getDifficulty() != null) {
             exerciseInfo.setDifficulty(this.getDifficulty());
+        } else {
+            exerciseInfo.setDifficulty("medium");
         }
         if (this.getEducationalAlignment() != null) {
             exerciseInfo.setEducationalAlignment(this.getEducationalAlignment());