test_host_tags: fix cleanup to handle None host tags

When a host has no explicit tags initially, cls.host.hosttags is None.
Passing None to the API does not set the hosttags parameter, so tags
are never cleared during cleanup. Convert None to empty string to properly
clear tags that were set during the test.

Author: weizhouapache

test/integration/smoke/test_host_tags.py

@@ -63,7 +63,7 @@ def tearDownClass(cls):
     def update_host_tags_via_api(cls, hosttags):
         cmd = updateHost.updateHostCmd()
         cmd.id = cls.host.id
-        cmd.hosttags = hosttags
+        cmd.hosttags = hosttags if hosttags is not None else ""
         cls.apiclient.updateHost(cmd)
 
     @classmethod