Just use the local conf variable

The local conf variable populated by the parent class
already ensures the none/falsey case and sets the conf
to an empty dictionary (as well as checks if its the
expected type and more...); so we should just use that
variable instead of duplicating that work.

Change-Id: I824263d5253d428f6a803f470e29d99de3dbf42c

taskflow/persistence/path_based.py

@@ -36,9 +36,7 @@ class PathBasedBackend(base.Backend):
 
     def __init__(self, conf):
         super(PathBasedBackend, self).__init__(conf)
-        if conf is None:
-                conf = {}
-        self._path = conf.get('path', None)
+        self._path = self._conf.get('path', None)
 
     @property
     def path(self):