input: Write a python function to find the sum of n natural numbers using recursion.
  assert sum_of_n_natural_numbers_recursive(10) == 55
  assert sum_of_n_natural_numbers_recursive(5) == 15
  assert sum_of_n_natural_numbers_recursive(0) == 0
output: def sum_of_n_natural_numbers_recursive(n):
      if n <= 0:
          return 0
      else:
          return n + sum_of_n_natural_numbers_recursive(n - 1)