input: Write a function to calculate the sum of an exponential series given a base `x` and the number of terms `n`. The function should return the sum of the series.
  assert exponential_series(2, 4) == 31
  assert exponential_series(3, 3) == 40
  assert exponential_series(1, 5) == 6
  assert exponential_series(0, 10) == 1
  assert exponential_series(5, 0) == 1
output: def exponential_series(x, n):
      if n == 0:
          return 1
      sum = 0
      for i in range(n):
          sum += x ** i
      return sum