Fix wf_trace info adding useless space at some conditions
Fix wf_trace info adding useless space at some conditions

when obj is None , debug_info is empty.
so there is no need to format msg using "%S %S", which is lead to adding a useless space at the end of msg.

Change-Id: I3d64c8b74e53ae4e11c614ca6fe27bdf4264fe77
Closes-Bug:#1732331

openstack/mistral,StackStorm/mistral,openstack/mistral,StackStorm/mistral