Fix error when no token is returned by Github

endpoints/callbacks.py

@@ -35,7 +35,11 @@ def exchange_github_code_for_token(code, for_login=True):
   get_access_token = client.post(app.config['GITHUB_TOKEN_URL'],
                                  params=payload, headers=headers)
 
-  token = get_access_token.json()['access_token']
+  json_data = get_access_token.json()
+  if not json_data:
+    return ''
+
+  token = json_data.get('access_token', '')
   return token