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. The function should iterate through each element in the list, check if the element is even, and add it to the result if it is even. Finally, return the sum.
  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, 0, 2, 4, -4])==0
  assert sum_even_numbers([0, 0, 0])==0
output: def sum_even_numbers(lst):
      return sum(x for x in lst if x % 2 == 0)