test: fix storage pool tags is not updated when test is done

Author: weizhouapache

test/integration/smoke/test_direct_download.py

@@ -251,7 +251,10 @@ def getCurrentStoragePoolTags(self, poolId):
             self.apiclient,
             id=poolId
         )
-        return local_pool[0].tags
+        if local_pool[0].tags:
+            return local_pool[0].tags
+        else:
+            return ""
 
     def updateStoragePoolTags(self, poolId, tags):
         StoragePool.update(