Nodes without any children at all (or empty graphs) have is_graph set
to FALSE. Therefore, the condition to handle non-meta-op graphs was
excluding such nodes. Trying to serialize such a node would lead to:
  CRITICAL **: gegl_node_get: assertion 'self->is_graph ||
    GEGL_IS_OPERATION (self->operation)' failed

Checking for the absence of a GeglOperation object covers both cases.
This is unlike gegl_node_get_operation which returns NULL for one and
"GraphNode" for another.

As a nice side-effect, this reverts to using only public API in
gegl_xml functions.

Fall out from 6a791c1f

https://bugzilla.gnome.org/show_bug.cgi?id=760161