UNOMI-651 : fix return type on groovy action executor -> do not return null (#483)

diff --git a/extensions/groovy-actions/services/src/main/java/org/apache/unomi/groovy/actions/GroovyActionDispatcher.java b/extensions/groovy-actions/services/src/main/java/org/apache/unomi/groovy/actions/GroovyActionDispatcher.java
index 8376813..dadeabb 100644
--- a/extensions/groovy-actions/services/src/main/java/org/apache/unomi/groovy/actions/GroovyActionDispatcher.java
+++ b/extensions/groovy-actions/services/src/main/java/org/apache/unomi/groovy/actions/GroovyActionDispatcher.java
@@ -74,6 +74,6 @@
                 logger.error("Error executing Groovy action with key=" + actionName, e);
             }
         }
-        return null;
+        return 0;
     }
 }