login: fix redirect loop when redirect_url="", yet set

Occured when logging in from homepage.

inventory-app/main.py

@@ -186,7 +186,7 @@ def name_check(form, field):
 @app.route("/login")
 @login_required
 def login_dummy():
-    return redirect(request.args.get("redirect_url", "/m/inventory"))
+    return redirect(request.args.get("redirect_url") or "/m/inventory")
 
 @app.route("/")
 def index():