Fix: Viewer node crashes when the editor is pinned and has no Object ID.

This case was not properly handled by the previous fix in #107621.

The viewer node path function still assumes that the node editor space
has a valid ID pointer (`snode.id`) and that it is an `Object`.
The ID pointer can be null when setting a pinned node tree through the
python API, like so:
```
space.pin = True
space.node_tree = some_other_node_tree
```

In this case the `ED_node_tree_start` function is called without and ID
pointer, which is allowed! The viewer node path function needs to handle
this case.

@@ -30,3 +29,1 @@
-  IDViewerPathElem *id_elem = BKE_viewer_path_elem_new_id();
-  id_elem->id = &ob->id;
-  BLI_addtail(&r_dst.path, id_elem);
+  if (snode.id != nullptr && GS(snode.id->name) == ID_OB) {

@@ -355,3 +360,3 @@
   ViewerPath tmp_viewer_path;
   BLI_SCOPED_DEFER([&]() { BKE_viewer_path_clear(&tmp_viewer_path); });
-  viewer_path_for_geometry_node(snode, *possible_viewer, tmp_viewer_path);
+  if (snode.id != nullptr && GS(snode.id->name) == ID_OB) {