nijie: fix exception in login process.

Fix an exception when we can't find the 'url' field in the login form
because we're rate limited by Nijie and couldn't scrape the login page.

app/logical/sources/strategies/nijie.rb

@@ -251,7 +251,7 @@ module Sources
         form = {
           email: Danbooru.config.nijie_login,
           password: Danbooru.config.nijie_password,
-          url: login_page.at("input[name='url']")["value"],
+          url: login_page.at("input[name='url']")&.fetch("value"),
           save: "on",
           ticket: ""
         }