ID Creation: generate `session_uuid` in more cases.

Previous code would not generate a session uuid for embedded IDs (like
root node trees or master collections). While this is not a problem
currently since those are not directly stored in Main data-base, this is
conceptually wrong, since those IDs still pertain the Main data.

Further more, this is blocking using `session_uuid` more in depsgraph in
place from ID pointer itself, as identifier (related to T84397).

source/blender/blenkernel/intern/lib_id.c

@@ -1087,8 +1087,6 @@ void *BKE_libblock_alloc(Main *bmain, short type, const char *name, const int fl
       /* alphabetic insertion: is in new_id */
       BKE_main_unlock(bmain);
 
-      BKE_lib_libblock_session_uuid_ensure(id);
-
       /* TODO to be removed from here! */
       if ((flag & LIB_ID_CREATE_NO_DEG_TAG) == 0) {
         DEG_id_type_tag(bmain, type);
@@ -1097,6 +1095,13 @@ void *BKE_libblock_alloc(Main *bmain, short type, const char *name, const int fl
     else {
       BLI_strncpy(id->name + 2, name, sizeof(id->name) - 2);
     }
+
+    /* We also need to ensure a valid `session_uuid` for some non-main data (like embedded IDs).
+     * IDs not allocated however should not need those (this would e.g. avoid generating session
+     * uuids for depsgraph CoW IDs, if it was using this function). */
+    if ((flag & LIB_ID_CREATE_NO_ALLOCATE) == 0) {
+      BKE_lib_libblock_session_uuid_ensure(id);
+    }
   }
 
   return id;