input: Write a function to calculate the sum of all even numbers in a given list of integers. If the list is empty, return 0.
  assert sum_even_numbers([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])==30
  assert sum_even_numbers([2, 4, 6, 8, 10])==30
  assert sum_even_numbers([1, 3, 5, 7, 9])==0
  assert sum_even_numbers([])==0
  assert sum_even_numbers([-2, -4, 0, 2, 4])==0
  assert sum_even_numbers([-2, -4, -6, -8])==-20
output: def sum_even_numbers(lst):
      return sum(x for x in lst if x % 2 == 0)