When eval-ing python expressions, return falsy but not-None values (m…

…ininet#1111)

mininet/cli.py

@@ -184,7 +184,7 @@ def do_py( self, line ):
  try:
  # pylint: disable=eval-used
  result = eval( line, globals(), self.getLocals() )
- if not result:
+ if result is None:
  return
  elif isinstance( result, str ):
  output( result + '\n' )