Fix: Geometry Nodes: crash with missing linked data-block

Geometry nodes can't be evaluated when the node tree is not available. This
never worked before and the modifier evaluation was "cancelled" later on before
already. However, new code in the modifier evaluation requires the modifier to
be cancelled earlier already which is what this patch does.

Pull Request: https://projects.blender.org/blender/blender/pulls/140923

source/blender/modifiers/intern/MOD_nodes.cc

@@ -1819,6 +1819,9 @@ static void modifyGeometry(ModifierData *md,
   }
   NodesModifierData *nmd_orig = reinterpret_cast<NodesModifierData *>(
       BKE_modifier_get_original(ctx->object, &nmd->modifier));
+  if (ID_MISSING(nmd_orig->node_group)) {
+    return;
+  }
 
   nodes::PropertiesVectorSet properties = nodes::build_properties_vector_set(
       nmd->settings.properties);