Do not download datastructure of datasets without data URL

download.py

@@ -132,8 +132,9 @@ def main():
             )
             continue
         dataset_url = leaf_element.findtext('./nt:downloadLink[@format="sdmx"]', namespaces=nsmap)
-        if dataset_url:
-            datasets.add((dataset_code, dataset_url))
+        if not dataset_url:
+            continue
+        datasets.add((dataset_code, dataset_url))
         datastructure_url = leaf_element.findtext('./nt:metadata[@format="sdmx"]', namespaces=nsmap)
         if datastructure_url:
             datastructure_code = datastructure_url.rsplit("/", 1)[-1].split(".", 1)[0]