input: Write a function to find the minimum value in a list of integers.
  assert find_minimum([3, 1, 4, 1, 5, 9, 2, 6, 5, 3, 5]) == 1
  assert find_minimum([10, 20, 30, 40, 50]) == 10
  assert find_minimum([-5, -10, 0, 10, 5]) == -10
output: def find_minimum(numbers):
      if not numbers:
          return None
      min_value = numbers[0]
      for num in numbers[1:]:
          if num < min_value:
              min_value = num
      return min_value