input: Write a function to find the maximum value within a nested dictionary. The function should recursively search through the dictionary and its nested dictionaries to find the maximum value. If the 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({})==None
  assert findMaxValue({"key1": 5, "key2": 10})==10
  assert findMaxValue({"key1": {"value1": 10, "value2": 20}, "key2": {"value3": 40}})==40
  assert findMaxValue({"a": 100, "b": {"c": 200, "d": 300}})==300
  assert findMaxValue({"a": {"b": {"c": 400, "d": 500}}, "e": 600})==600
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