input: Write a python function to read a JSON file and return its content as a dictionary.
  assert read_json_file('data.json') == {"countries": ["USA", "Canada", "Mexico"]}
  assert read_json_file('empty.json') == {}
  assert read_json_file('invalid.json') == None
output: def read_json_file(filename):
      import json
      try:
          with open(filename, 'r') as file:
              return json.load(file)
      except (FileNotFoundError, json.JSONDecodeError):
          return None