input: Write a function to calculate the sum of the even numbers in a given list of integers. Return the sum of the even numbers. If the given list is empty, return 0.
  assert sum_even_numbers([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]) == 30
  assert sum_even_numbers([1, 3, 5, 7, 9]) == 0
  assert sum_even_numbers([2, 4, 6, 8, 10]) == 30
  assert sum_even_numbers([]) == 0
  assert sum_even_numbers([-2, -4, -6, -8, -10]) == -30
  assert sum_even_numbers([0]) == 0
output: def sum_even_numbers(numbers):
      return sum(num for num in numbers if num % 2 == 0)