input: Write a function that improves the given function by using Python built-in functions to take an array of numbers and return the sum of all the numbers. If the given array is empty, it should return 0.
  assert sum_array([1, 2, 3, 4, 5])==15
  assert sum_array([10, 20, 30])==60
  assert sum_array([-1, -2, -3, -4, -5])==-15
  assert sum_array([])==0
  assert sum_array([0])==0
output: def sum_array(arr):
      return sum(arr) if arr else 0