set empty command_string in enter cmd
@@ -45,7 +45,7 @@ class Command(ConnectionObserver):
# check base class invariants first
super(Command, self)._validate_start(*args, **kwargs)
# then what is needed for command
- if not self.command_string:
+ if self.command_string is None:
# no chance to start CMD
raise NoCommandStringProvided(self)
