input: Write a function to print the Fibonacci series up to n numbers where n is a user input. 
  assert fibonacci_series(10)==0 1 1 2 3 5 8 13 21 34
  assert fibonacci_series(5)==0 1 1 2 3
  assert fibonacci_series(1)==0
  assert fibonacci_series(0)==
  assert fibonacci_series(15)==0 1 1 2 3 5 8 13 21 34 55 89 144 233 377
output: def fibonacci_series(n):
      if n == 0:
          return ""
      if n == 1:
          return "0"
      result = "0 1"
      a, b = 0, 1
      for _ in range(2, n):
          a, b = b, a + b
          result += f" {b}"
      return result