Fix empty group in rax.py output

plugins/inventory/rax.py

@@ -168,6 +168,7 @@ def _list(regions):
                 groups[group].append(server.name)
 
             for extra_group in server.metadata.get('groups', '').split(','):
+                if extra_group:
                     groups[extra_group].append(server.name)
 
             # Add host metadata