input: Write a python function to print the Fibonacci series up to n numbers.
  assert fibonacci_series(5) == "0 1 1 2 3"
  assert fibonacci_series(8) == "0 1 1 2 3 5 8 13"
  assert fibonacci_series(3) == "0 1 1"
output: def fibonacci_series(n):
      if n <= 0:
          return ""
      series = []
      a, b = 0, 1
      for _ in range(n):
          series.append(str(a))
          a, b = b, a + b
      return " ".join(series)