Use none for no previous ensemble id

Author: tbwiik

src/ert/gui/experiments/multiple_data_assimilation_panel.py

@@ -62,7 +62,7 @@ class Arguments:
     target_ensemble: str
     realizations: str
     weights: str
-    prior_ensemble_id: str  # UUID not serializable in json
+    prior_ensemble_id: str | None  # UUID not serializable in json
     experiment_name: str
 
 
@@ -425,7 +425,7 @@ def get_experiment_arguments(self) -> Arguments:
                 str(self._ensemble_selector.selected_ensemble.id)
                 if self._ensemble_selector.selected_ensemble is not None
                 and self._select_prior_ensemble_box.isChecked()
-                else ""
+                else None
             ),
             experiment_name=self._experiment_name_field.get_text,
         )

src/ert/run_models/model_factory.py

@@ -467,7 +467,7 @@ def _determine_previous_ensemble_id(args: Namespace) -> str | None:
     """
     if hasattr(args, "restart_ensemble_id"):
         # When running from CLI
-        prior_ensemble = args.restart_ensemble_id or ""
+        prior_ensemble = args.restart_ensemble_id or None
     else:
         # When running from GUI
         prior_ensemble = args.prior_ensemble_id