input: Write a function to print out 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"
output: def fibonacci_series(n):
      if n <= 0:
          return ""
      elif n == 1:
          return "0"
      elif n == 2:
          return "0 1"
      
      fib_series = [0, 1]
      for i in range(2, n):
          fib_series.append(fib_series[-1] + fib_series[-2])
      
      return " ".join(map(str, fib_series))