input: Write a function that creates a Python class which represents a 3D Vector with methods to initialize the vector and return its string representation. If the given vector is empty then return an empty string.
  
  assert Vector3D(3, 4, 5).__str__()=="(3, 4, 5)"
  assert Vector3D(0, 0, 0).__str__()=="(0, 0, 0)"
  assert Vector3D(-1, -1, -1).__str__()=="(-1, -1, -1)"
  assert Vector3D(1.5, 2.5, 3.5).__str__()=="(1.5, 2.5, 3.5)"
  assert Vector3D(None, None, None).__str__()=="(None, None, None)"
  assert Vector3D(0, 1, 0).__str__()=="(0, 1, 0)"
output: class Vector3D:
      def __init__(self, x, y, z):
          self.x = x
          self.y = y
          self.z = z
  
      def __str__(self):
          if self.x is None and self.y is None and self.z is None:
              return ""
          return f"({self.x}, {self.y}, {self.z})"