input: Write a python function to find the largest number in an array.
  assert get_largest_number([1, 2, -4, 8, 3, 7]) == 8
  assert get_largest_number([10, 20, 45, 67, 84, 93]) == 93
  assert get_largest_number([-5, -7, -9, -8, -6, -4, -3]) == -3
output: def get_largest_number(arr):
      if not arr:
          return None
      largest = arr[0]
      for num in arr:
          if num > largest:
              largest = num
      return largest