input: Write a python function to convert a list of keys into a dictionary with default values as None.
  assert keys_to_dict(["Stud1", "Roll No", "Class", "Stud2"]) == {"Stud1": None, "Roll No": None, "Class": None, "Stud2": None}
  assert keys_to_dict(["Name", "Age", "Gender"]) == {"Name": None, "Age": None, "Gender": None}
  assert keys_to_dict([]) == {}
output: def keys_to_dict(keys):
      return {key: None for key in keys}