input: Write a function to download subtitles for movies and TV shows given one or more release paths, along with optional language and subtitle site specifications. If the download is successful, return the path of the subtitle file, otherwise return None. If no release paths are provided, return a message indicating no release paths were provided.
  
  assert download_subtitles(["/path/to/movie1", "/path/to/movie2"], ["opensubtitles"], "en") == ["/path/to/subtitle_file1", "/path/to/subtitle_file2"]
  assert download_subtitles(["/path/to/not_found_movie"], ["opensubtitles"], "en") == [None]
  assert download_subtitles([], ["opensubtitles"], "en") == "No release paths provided."
  assert download_subtitles(["/path/to/movie3"], [], "en") == ["/path/to/subtitle_file3"]
  
output: def download_subtitles(release_paths, subtitle_sites=[], language=""):
      if not release_paths:
          return "No release paths provided."
      
      subtitle_files = []
      for path in release_paths:
          if any(site in path for site in subtitle_sites):
              subtitle_files.append(f"{path}_subtitle_file")
          else:
              subtitle_files.append(None)
      
      return subtitle_files