Migrate v5 storages again

src/ert/storage/local_storage.py

@@ -305,6 +305,7 @@ def _migrate(self, ignore_migration_check: bool) -> None:
             ert_kind,
             experiment_id,
             gen_kw,
+            metadata,
             observations,
             response_info,
             update_params,
@@ -347,6 +348,11 @@ def _migrate(self, ignore_migration_check: bool) -> None:
                 update_params.migrate(self.path)
                 empty_summary.migrate(self.path)
                 self._add_migration_information(4, "experiment_id")
+            elif version == 5:
+                # Should usually not need to migrate here, but
+                # there was a bug on a stable release, which left
+                # some storage partially migrated
+                metadata.migrate(self.path)
         except Exception as err:  # pylint: disable=broad-exception-caught
             logger.error(f"Migrating storage at {self.path} failed with {err}")
 

src/ert/storage/migration/metadata.py

@@ -0,0 +1,14 @@
+from __future__ import annotations
+
+import json
+from typing import TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from pathlib import Path
+
+
+def migrate(path: Path) -> None:
+    for experiment in path.glob("experiments/*"):
+        if not (experiment / "metadata.json").exists():
+            with open(experiment / "metadata.json", "w", encoding="utf-8") as fout:
+                fout.write(json.dumps({}))