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(0)==""
  assert fibonacci_series(1)=="0 "
  assert fibonacci_series(5)=="0 1 1 2 3 "
  assert fibonacci_series(1)=="0 "
output: def fibonacci_series(n):
      if n == 0:
          return ""
      result = "0 "
      a, b = 0, 1
      for _ in range(1, n):
          result += str(b) + " "
          a, b = b, a + b
      return result