Fix references to some nodegroups from new essentials staying around.

Previous fix/hack a60f65150267  made it so that all 'Fake User' linked
IDs are considered as directly linked, so that some IDs (like linked
Text) references are not lost on file save becaus they are not
effectively used.

However, this created unwanted references to all linked assets data,
because an asset always has its fake user set. This ended up keeping
lots of references to sub-assets when appending from the new Essentials
node groups.

NOTE: This is adding another hack to the collection, the whole handling
of 'which unused linked data reference to keep on file save' needs to be
reworked for Blender 3.6.

source/blender/blenloader/intern/writefile.cc

@@ -1141,7 +1141,7 @@ static bool write_file_handle(Main *mainvar,
            * asap afterward. */
           id_lib_extern(id_iter);
         }
-        else if (ID_FAKE_USERS(id_iter) > 0) {
+        else if (ID_FAKE_USERS(id_iter) > 0 && id_iter->asset_data == nullptr) {
           /* Even though fake user is not directly editable by the user on linked data, it is a
            * common 'work-around' to set it in library files on data-blocks that need to be linked
            * but typically do not have an actual real user (e.g. texts, etc.).