plugins exts UPDATE ext callback for data node validation

Fixes #1908
diff --git a/src/plugins_exts/metadata.c b/src/plugins_exts/metadata.c
index eebb434..8326f6b 100644
--- a/src/plugins_exts/metadata.c
+++ b/src/plugins_exts/metadata.c
@@ -163,6 +163,7 @@
         .plugin.compile = &annotation_compile,
         .plugin.sprinter = &annotation_schema_printer,
         .plugin.free = annotation_free,
+        .plugin.node = NULL,
         .plugin.snode = NULL,
         .plugin.validate = NULL
     },