input: Write a python function to check if one list is a subset of another list.
  assert is_subset([1, 2, 3], [1, 2, 3, 4, 5]) == True
  assert is_subset([1, 2, 6], [1, 2, 3, 4, 5]) == False
  assert is_subset([], [1, 2, 3]) == True
  assert is_subset([1, 2, 3], []) == False
output: def is_subset(list1, list2):
      return all(item in list2 for item in list1)