input: Write a function to determine if it is possible to compare two strings and return whether they are equal, different, or similar up to a provided threshold. If the given strings are empty then return None.
  class StringComparator:
  
  def __init__(self, string1, string2):
   self.string1 = string1
   self.string2 = string2
  
  def is_equal(self):
   return self.string1 == self.string2
  
  def is_different(self):
   return self.string1 != self.string2
  
  def is_similar(self, threshold=0.5):
   if not self.string1 or not self.string2:
   return None
   distance = edit_distance(self.string1, self.string2)
   max_length = max(len(self.string1), len(self.string2))
  
   return distance / max_length < threshold
  
  assert StringComparator("hello", "hello").is_equal()==True
  assert StringComparator("hello", "hell").is_equal()==False
  assert StringComparator("", "").is_similar()==None
  assert StringComparator("abc", "ab").is_similar(threshold=0.3)==False
  assert StringComparator("abcdef", "abcdeg").is_similar(threshold=0.1)==True
output: 
  def is_similar(self, threshold=0.5):
   if not self.string1 or not self.string2:
    return None
   distance = edit_distance(self.string1, self.string2)
   max_length = max(len(self.string1), len(self.string2))
   return distance / max_length < threshold
  