input: Write a function to find the maximum value within a nested dictionary. If the dictionary is empty or contains no values, the function should return None.
  assert findMaxValue({})==None
  assert findMaxValue({"key1": 10, "key2": 20})==20
  assert findMaxValue({"key1": {"value1": 10, "value2": 20}, "key2": {"value3": 40}})==40
  assert findMaxValue({"key1": {"value1": {"value2": 30}, "value3": 25}, "key2": 15})==30
  assert findMaxValue({"key1": {"value1": 10, "value2": 20}, "key2": {"value3": {"value4": 100}}})==100
output: def findMaxValue(d):
      if not d:
          return None
      max_value = None
      stack = [d]
      while stack:
          current = stack.pop()
          for key, value in current.items():
              if isinstance(value, dict):
                  stack.append(value)
              else:
                  if max_value is None or value > max_value:
                      max_value = value
      return max_value