Why should functions always return the same type?

I read somewhere that functions should always return only one type so the following code is considered as bad code:

def x(foo):
 if 'bar' in foo:
  return (foo, 'bar')
 return None

I guess the better solution would be

def x(foo):
 if 'bar' in foo:
  return (foo, 'bar')
 return ()

Wouldn't it be cheaper memory wise to return a None then to create a new empty tuple or is this time difference too small to notice even in larger projects?