Fixed bug where self.persistent_history_file attribute didn't exist

Author: kmvanbrunt

cmd2/cmd2.py

@@ -441,6 +441,7 @@ def __init__(self, completekey: str='tab', stdin=None, stdout=None, persistent_h
         self.broken_pipe_warning = ''
 
         # Check if history should persist
+        self.persistent_history_file = ''
         if persistent_history_file and rl_type != RlType.NONE:
             persistent_history_file = os.path.expanduser(persistent_history_file)
             read_err = False