input: Write a function to get weather data using latitude and longitude.
  assert get_weather_data('40.7128', '-74.0060') is not None
  assert get_weather_data('34.0522', '-118.2437') is not None
  assert get_weather_data('', '') is None
output: import requests
  
  def get_weather_data(lat, lon):
      if not lat or not lon:
          return None
      api_key = 'YOUR_API_KEY_HERE'
      url = f'http://api.openweathermap.org/data/2.5/weather?lat={lat}&lon={lon}&appid={api_key}'
      response = requests.get(url)
      if response.status_code == 200:
          return response.json()
      return None