Add nullability constraints everywhere.

Change-Id: I80577b7784006070f29ce11af4e6fd87c10363bf
diff --git a/src/main/java/eu/mulk/mulkcms2/cms/pages/ArticleRevisionCharacteristic.java b/src/main/java/eu/mulk/mulkcms2/cms/pages/ArticleRevisionCharacteristic.java
index d5ca828..0ac7ae5 100644
--- a/src/main/java/eu/mulk/mulkcms2/cms/pages/ArticleRevisionCharacteristic.java
+++ b/src/main/java/eu/mulk/mulkcms2/cms/pages/ArticleRevisionCharacteristic.java
@@ -1,6 +1,7 @@
 package eu.mulk.mulkcms2.cms.pages;
 
 import io.quarkus.hibernate.orm.panache.PanacheEntityBase;
+import javax.annotation.CheckForNull;
 import javax.persistence.Column;
 import javax.persistence.Entity;
 import javax.persistence.FetchType;
@@ -33,5 +34,6 @@
   public ArticleRevision articleRevision;
 
   @Column(name = "value", nullable = true, length = -1)
+  @CheckForNull
   public String value;
 }