input: Write a python function to calculate the sum of the first n natural numbers using a while loop.
  assert sum_natural_numbers(10) == 55
  assert sum_natural_numbers(5) == 15
  assert sum_natural_numbers(0) == 0
output: def sum_natural_numbers(n):
      sum = 0
      i = 1
      while i <= n:
          sum += i
          i += 1
      return sum