input: Write a function to find the maximum value within a nested dictionary. If the input dictionary is empty, the function should return None.
  
  Output:
  def findMaxValue(dict):
      if not dict:
          return None
      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": 10,
          "value2": {
              "value3": 50,
              "value4": 30
          }
      },
      "key2": {
          "value5": 40
      },
  }) == 50
  assert findMaxValue({"key": 10}) == 10
  assert findMaxValue({}) == None
output: def findMaxValue(dict):
      if not dict:
          return None
      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