agent: empty list in AGENT_HOST_PATTERNS means nothing for the service (#5875)

hobo/agent/services.py

@@ -30,7 +30,7 @@ class BaseService(object):
         if not settings.AGENT_HOST_PATTERNS:
             return True
         patterns = settings.AGENT_HOST_PATTERNS.get(self.service_id)
-        if not patterns:
+        if patterns is None:
             return True
         parsed_url = urllib2.urlparse.urlsplit(self.base_url)
         netloc = parsed_url.netloc