input: Write a function to find the maximum value within a nested dictionary. If the input dictionary is empty, return None.
  def findMaxValue(dict):
      max_val = None
      for key, value in dict.items():
          if isinstance(value, dict):
              max_val = max(findMaxValue(value), max_val)
          else:
              max_val = max(value, max_val)
      return max_val
  
  assert findMaxValue({"key1": {"value1": 10, "value2": 20}, "key2": {"value3": 40}}) == 40
  assert findMaxValue({"key1": {"value1": 5, "value2": 8}, "key2": {"value3": 12}}) == 12
  assert findMaxValue({"key1": 3, "key2": 5, "key3": 7}) == 7
  assert findMaxValue({}) == None
  assert findMaxValue({"key1": {}, "key2": {}}) == None
output: def findMaxValue(dict):
      max_val = None
      for key, value in dict.items():
          if isinstance(value, dict):
              max_val = max(findMaxValue(value), max_val)
          else:
              max_val = max(value, max_val)
      return max_val