input: Write a python function to convert a list to a dictionary with None as values.
  assert list_to_dict(['a', 'b', 'c']) == {'a': None, 'b': None, 'c': None}
  assert list_to_dict([1, 2, 3, 4]) == {1: None, 2: None, 3: None, 4: None}
  assert list_to_dict([]) == {}
output: def list_to_dict(lst):
      return {item: None for item in lst}