Fix multiple bugs and code smells

This fixes multiple blocker and critical rated issues reported by SonarQube.

src/main/java/de/thm/arsnova/cache/CacheBusterImpl.java

@@ -39,36 +39,36 @@ public class CacheBusterImpl implements CacheBuster {
 	@CacheEvict(value = "statistics", allEntries = true)
 	@EventListener
 	public void handleAfterCommentCreation(final AfterCreationEvent<Comment> event) {
-
+		/* Implementation provided by caching aspect. */
 	}
 
 	@CacheEvict(value = "statistics", allEntries = true)
 	@EventListener
 	public void handleAfterCommentDeletion(final AfterDeletionEvent<Comment> event) {
-
+		/* Implementation provided by caching aspect. */
 	}
 
 	@CacheEvict(value = "answerlists", key = "#event.content.id")
 	@EventListener
 	public void handleAfterAnswerCreation(final AfterCreationEvent<Answer> event) {
-
+		/* Implementation provided by caching aspect. */
 	}
 
 	@CacheEvict(value = "statistics", allEntries = true)
 	@EventListener
 	public void handleChangeScore(final ChangeScoreEvent event) {
-
+		/* Implementation provided by caching aspect. */
 	}
 
 	@CacheEvict(value = "statistics", allEntries = true)
 	@EventListener
 	public void handleAfterRoomCreation(final AfterCreationEvent<Room> event) {
-
+		/* Implementation provided by caching aspect. */
 	}
 
 	@CacheEvict(value = "statistics", allEntries = true)
 	@EventListener
 	public void handleAfterRoomDeletion(final AfterDeletionEvent<Room> event) {
-
+		/* Implementation provided by caching aspect. */
 	}
 }