Properly marks field as nullable

- see scireum/sirius-db#647

Fixes: SIRI-939

src/main/java/sirius/biz/tycho/kb/KnowledgeBaseEntry.java

@@ -94,6 +94,7 @@ public class KnowledgeBaseEntry extends SearchableEntity {
  * Contains a short and concise description of this article.
  */
  public static final Mapping DESCRIPTION = Mapping.named("description");
+ @NullAllowed
  @SearchContent
  private String description;