Fix: Nodes: handle undefined nodes more gracefully

Found while checking #134193.

source/blender/blenkernel/BKE_node_runtime.hh

@@ -822,6 +822,11 @@ inline bool bNode::is_group_output() const
   return this->type_legacy == NODE_GROUP_OUTPUT;
 }
 
+inline bool bNode::is_undefined() const
+{
+  return this->typeinfo == &blender::bke::NodeTypeUndefined;
+}
+
 inline bool bNode::is_type(const blender::StringRef query_idname) const
 {
   return this->typeinfo->is_type(query_idname);

source/blender/makesdna/DNA_node_types.h

@@ -492,6 +492,7 @@ typedef struct bNode {
   bool is_group() const;
   bool is_group_input() const;
   bool is_group_output() const;
+  bool is_undefined() const;
 
   /**
    * Check if the node has the given idname.

source/blender/nodes/intern/socket_usage_inference.cc

@@ -148,6 +148,10 @@ struct SocketUsageInferencer {
     if (all_socket_usages_.contains(socket)) {
       return;
     }
+    if (socket->owner_node().is_undefined()) {
+      all_socket_usages_.add_new(socket, false);
+      return;
+    }
     if (socket->is_input()) {
       this->usage_task__input(socket);
     }
@@ -481,6 +485,10 @@ struct SocketUsageInferencer {
       /* Task is done already. */
       return;
     }
+    if (socket->owner_node().is_undefined()) {
+      all_socket_values_.add_new(socket, nullptr);
+      return;
+    }
     const CPPType *base_type = socket->typeinfo->base_cpp_type;
     if (!base_type) {
       /* The socket type is unknown for some reason (maybe a socket type from the future?).*/