Fix: Always add timestamp_raw attributes to xarray metadata

- Changed _add_metadata_to_xarray to always add run_timestamp_raw and completed_timestamp_raw
- Removes conditional logic that only added these when not None
- Now consistent with run_timestamp and completed_timestamp behavior
- Ensures consistency between export and load methods

Fixes #7330

Co-authored-by: jenshnielsen <548266+jenshnielsen@users.noreply.github.com>

Author: Copilot

src/qcodes/dataset/exporters/export_to_xarray.py

@@ -217,10 +217,8 @@ def _add_metadata_to_xarray(
             "parent_dataset_links": links_to_str(dataset.parent_dataset_links),
         }
     )
-    if dataset.run_timestamp_raw is not None:
-        xrdataset.attrs["run_timestamp_raw"] = dataset.run_timestamp_raw
-    if dataset.completed_timestamp_raw is not None:
-        xrdataset.attrs["completed_timestamp_raw"] = dataset.completed_timestamp_raw
+    xrdataset.attrs["run_timestamp_raw"] = dataset.run_timestamp_raw
+    xrdataset.attrs["completed_timestamp_raw"] = dataset.completed_timestamp_raw
     if len(dataset.metadata) > 0:
         for metadata_tag, metadata in dataset.metadata.items():
             xrdataset.attrs[metadata_tag] = metadata