Fix T96402: fix case when material output is contained in node group

For now just assume that a node group without output sockets is
an output node. Ideally, we would use run-time information stored
on the node group itself to determine if the group contains a
top-level output node (e.g. Material Output). That can be
implemented separately.

In the larger scheme of things, top-level outputs within node
groups seem to break the node group abstraction and reusability
a bit.

source/blender/blenkernel/intern/node_tree_update.cc

@@ -1353,8 +1353,7 @@ class NodeTreeMainUpdater {
   {
     Vector<const SocketRef *> sockets;
     for (const NodeRef *node : tree.nodes()) {
-      const bNode *bnode = node->bnode();
-      if (bnode->typeinfo->nclass != NODE_CLASS_OUTPUT && bnode->type != NODE_GROUP_OUTPUT) {
+      if (!this->is_output_node(*node)) {
         continue;
       }
       for (const InputSocketRef *socket : node->inputs()) {
@@ -1366,6 +1365,24 @@ class NodeTreeMainUpdater {
     return sockets;
   }
 
+  bool is_output_node(const NodeRef &node) const
+  {
+    const bNode &bnode = *node.bnode();
+    if (bnode.typeinfo->nclass == NODE_CLASS_OUTPUT) {
+      return true;
+    }
+    if (bnode.type == NODE_GROUP_OUTPUT) {
+      return true;
+    }
+    /* Assume node groups without output sockets are outputs. */
+    /* TODO: Store whether a node group contains a top-level output node (e.g. Material Output) in
+     * run-time information on the node group itself. */
+    if (bnode.type == NODE_GROUP && node.outputs().is_empty()) {
+      return true;
+    }
+    return false;
+  }
+
   /**
    * Computes a hash that changes when the node tree topology connected to an output node changes.
    * Adding reroutes does not have an effect on the hash.