Fix bug with fetching contributors twice for guides that do not have any

- This bug increased our API usage because it requested the list of
  contributors twice instead of only once for guides that did not have any
  contributors.  The underlying Article object had no distinction between the
  two states, 'requested and found no contributors' and 'have not requested
  yet'.  So we would send 2 extra API requests for every guide that did not
  have contributors, which is most likely the majority of our guides at this
  point.
    - The reason it takes 2 requests to get the contributors is we have to look
      in the published and in-review directories since that's how the API
      tracks commits.  The github API doesn't support the --follow parameter of
      command-line git.

pskb_website/models/article.py

@@ -897,7 +897,7 @@ def __init__(self, title, author_name, filename=ARTICLE_FILENAME,
 
         # List of User objects representing any 'author' i.e user who has
         # contributed at least 1 line of text to this article.
-        self._contributors = []
+        self._contributors = None
 
     @property
     def path(self):
@@ -945,7 +945,7 @@ def contributors(self):
         # NOTE: This could result in some data out of data if we have new
         # contributors after this is called but contributor information isn't
         # super important so should be ok.
-        if self._contributors:
+        if self._contributors is not None:
             return self._contributors
 
         self._read_contributors_from_api()