input: Write a python function to convert a list of tuples into a dictionary, skipping tuples where the second element is None.
  assert tuple_to_dict([(1, 2), (3, 4), (5, None)])=={1: 2, 3: 4}
  assert tuple_to_dict([(2, 3), (4, None), (6, 7), (8, None)])=={2: 3, 6: 7}
  assert tuple_to_dict([(9, 10), (11, 12), (13, None)])=={9: 10, 11: 12}
output: def tuple_to_dict(tuples):
      return {k: v for k, v in tuples if v is not None}