Fix #33800: GLSL group nodes not using the external input value unless a node

was connected to the socket.

source/blender/nodes/shader/node_shader_util.c

@@ -221,6 +221,7 @@ void node_gpu_stack_from_data(struct GPUNodeStack *gs, int type, bNodeStack *ns)
 
 void node_data_from_gpu_stack(bNodeStack *ns, GPUNodeStack *gs)
 {
+	copy_v4_v4(ns->vec, gs->vec);
 	ns->data= gs->link;
 	ns->sockettype= gs->sockettype;
 }