Core: fix unfilled items "Per-Player Count"

[Silvris]

What is this fixing or adding?

At some point (likely the region caching rework), multiworld.get_locations was changed to include unfilled locations as well as filled, creating a mismatch between the evaluated per-player count and the actual player location count.

How was this tested?

Debugging two plando bugs, and wondering why the counts were showing an additional 2 locations.

If this makes graphical changes, please attach screenshots.

[Silvris]

The unfilled locations are already accounted for within .get_locations(), we don't need to duplicate them. I wasn't actually sure that it used to be filled only, I just assumed that to be the case since it would justify the existence of adding the unfilled. We have to update the items, as since these items remained unplaced, they have no ties to any location.

[qwint]

not certain about the code changes but they seem simple enough but I did test both main and this branch using my world (minit) to submit too many items and not enough items and see the messaging, which seems to replicate the issue Silvris is trying to solve and show it corrected on the PR's branch too few items on main (6 too few), the Counters misrepresent unfilled locations

too few items on this pr (6 too few), the Counters match the rest of the info

too many items on main (9 too many), no issue

too many items on this pr (9 too many), no change (so no issue)