On the production, yous should set it to "" if you don't want to use a different login page, but the standard one. I've also had to look a long time for this!
From the documentation on the constant: If login is required, this constant defines where the user can enter his credentials. If empty, the default built-in login page is used.
If not empty, it is assumed that after login, the user will be redirected to the deeplink again. For this reason the provided url is appended with the original deeplink.
For example: 'https://mxid.mendix.com/login?a=MyApp&f=true&cont=' or '../sso/login?f=true&cont='
So you could leave it empty to use the standard login page else the page needs to be defined and the url part used for the deppelingk will be appended to the entered url.