raise error when required username or password are missing

do not try to login as 'None' (#1192)

gallery_dl/extractor/nijie.py

@@ -100,6 +100,10 @@ class NijieExtractor(AsynchronousMixin, Extractor):
 
     @cache(maxage=150*24*3600, keyarg=1)
     def _login_impl(self, username, password):
+        if not username or not password:
+            raise exception.AuthenticationError(
+                "Username and password required")
+
         self.log.info("Logging in as %s", username)
         url = "{}/login_int.php".format(self.root)
         data = {"email": username, "password": password, "save": "on"}

gallery_dl/extractor/pixiv.py

@@ -522,6 +522,10 @@ class PixivAppAPI():
 
     @cache(maxage=3600, keyarg=1)
     def _login_impl(self, username, password):
+        if not username or not password:
+            raise exception.AuthenticationError(
+                "Username and password required")
+
         url = "https://oauth.secure.pixiv.net/auth/token"
         data = {
             "client_id": self.client_id,

gallery_dl/extractor/seiga.py

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright 2016-2019 Mike Fährmann
+# Copyright 2016-2020 Mike Fährmann
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License version 2 as
@@ -54,6 +54,10 @@ class SeigaExtractor(Extractor):
 
     @cache(maxage=7*24*3600, keyarg=1)
     def _login_impl(self, username, password):
+        if not username or not password:
+            raise exception.AuthenticationError(
+                "Username and password required")
+
         self.log.info("Logging in as %s", username)
         url = "https://account.nicovideo.jp/api/v1/login"
         data = {"mail_tel": username, "password": password}