input: Write a function to find the maximum value within a nested dictionary. The function should recursively search through all levels of the dictionary and return the highest value encountered. If the dictionary is empty, the function should 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
  
  sample_dict = {
      "key1": {
          "value1": 10,
          "value2": 20
      },
      "key2": {
          "value3": 40
      },
  }
  
  assert findMaxValue(sample_dict)==40
  assert findMaxValue({})==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