input: Write a python function to fetch and return the names and capitals of all countries using the given API.
  assert fetch_country_capitals()[:3] == [('Afghanistan', 'Kabul'), ('Åland Islands', 'Mariehamn'), ('Albania', 'Tirana')]
  assert isinstance(fetch_country_capitals(), list)
  assert len(fetch_country_capitals()) > 0
output: def fetch_country_capitals():
      import requests
      
      url = "https://restcountries.com/v3.1/all"
      response = requests.get(url)
      countries = response.json()
      
      country_capitals = [(country['name']['common'], country['capital'][0] if 'capital' in country and country['capital'] else None) for country in countries]
      
      return country_capitals