Fix: Missing upstream snapshot in the v0023 migration script (#1425)

Author: izeigerman

sqlmesh/migrations/v0023_fix_added_models_with_forward_only_parents.py

@@ -45,6 +45,8 @@ def migrate(state_sync: t.Any) -> None:
         has_paused_forward_only_parent = False
         if is_breaking and not has_previous_versions:
             for upstream_id in dag.upstream(snapshot_id):
+                if upstream_id not in snapshot_mapping:
+                    continue
                 upstream_snapshot = snapshot_mapping[upstream_id]
                 upstream_change_category = upstream_snapshot.get("change_category")
                 is_forward_only_upstream = upstream_change_category == 3