Fix #147556: missing volume grid output when accessing subframe of bake

The code attempted to mix the grids as single values. In theory, we could
attempt interpolating between grids, but that can be done separately.

Pull Request: https://projects.blender.org/blender/blender/pulls/147557

source/blender/nodes/geometry/nodes/node_geo_simulation.cc

@@ -939,7 +939,7 @@ void mix_baked_data_item(const eNodeSocketDatatype socket_type,
     case SOCK_RGBA:
     case SOCK_MATRIX: {
       const CPPType &type = *bke::socket_type_to_geo_nodes_base_cpp_type(socket_type);
-      if (prev.is_context_dependent_field() || next.is_context_dependent_field()) {
+      if (!prev.is_single() || !next.is_single()) {
         /* Fields are evaluated on geometries and are mixed there. */
         break;
       }