Cast TISS ID into a string for Invenio-DAMAP

* the identifier is expected to be a string, but the TISS ID would
  typically be an integer which caused issues with the integration

invenio_config_tuw/dmp.py

@@ -29,6 +29,6 @@ def tuw_id_generator(identity) -> dict[str, str]:
             tiss_id = u.user_profile.get("tiss_id")
 
             if tiss_id:
-                identifiers["tiss_id"] = tiss_id
+                identifiers["tiss_id"] = str(tiss_id)
 
     return identifiers