input: Write a function to generate a Python class to represent a 3-dimensional vector with an initializer to set the x, y, and z coordinates, and a method to calculate the length of the vector. If the given input is empty then return an empty class.
  
  class Vector3D:
      def __init__(self, x, y, z):
          self.x = x
          self.y = y
          self.z = z
      
      def length(self):
          return (self.x**2 + self.y**2 + self.z**2)**0.5
  
  assert Vector3D(3, 4, 5).length()==7.0710678118654755
  assert Vector3D(1, 1, 1).length()==1.7320508075688772
  assert Vector3D(0, 0, 0).length()==0.0
  assert Vector3D(2, 2, 2).length()==3.4641016151377544
  assert Vector3D(6, 8, 10).length()==14.142135623730951
output: class Vector3D:
      def __init__(self, x, y, z):
          self.x = x
          self.y = y
          self.z = z
      
      def length(self):
          return (self.x**2 + self.y**2 + self.z**2)**0.5