dubious $null in user_register() function (line 1190 of user.module)

Not a bug really, but questionable code that could become a bug in the future ...

Here's the offending line 1190 of user.module (ver. 4.7.1)

$extra = _user_forms($null, $null, $null, 'register');

It's part of the user_register() callback that displays a registration form. This invocation of _user_forms gets its value in the return of $extra instead of modifying the first argument (which is passed by reference). In this case, $null isn't set or referenced anywhere else on the site so it works, but I'd think that at minimum the second two '$null' values would better be NULL, and the first one named something less confusing (e.g., ignoreme).

I don't see the use of $null anywhere else in the modules code, so I presume this wasn't a coding convention that anyone actually intended ...

As a side note - the use of this user_register() function is interesting in that it behaves differently if called from different urls. In particular, the profile module only displays its custom profile fields when users register on the normal user/register page, but the og module offers it's subscription links regardless. Kind of funky and obscure, really. In my case, I'm using the theme_user_register mechanism to make registration on a group page ask for different info from a full-on site registration. I'd be curious if others have already addressed this problem in a different way. Makes me dream of a more sophisticated registration code interface (notice I didn't call it an api!).