[637] Fix NPE when actionHistory is null (#638)

Signed-off-by: Domenico Briganti <briganti.domenico@gmail.com>

hawkbit-ddi-api/src/main/java/org/eclipse/hawkbit/ddi/json/model/DdiDeploymentBase.java

@@ -76,7 +76,7 @@ public class DdiDeploymentBase extends ResourceSupport {
     @Override
     public String toString() {
         return "DeploymentBase [id=" + deplyomentId + ", deployment=" + deployment + " actionHistory="
-                + actionHistory.toString() + "]";
+                + actionHistory + "]";
     }
 
 }