input: Write a python function to find the minimum value in a list of numbers.
  assert find_minimum([24, 14, 67, 89, 34, 52]) == 14
  assert find_minimum([10, 20, 30, 5, 25]) == 5
  assert find_minimum([7, 7, 7, 7, 7]) == 7
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