The PlaceBlock config action doesn't account for the possibility of there being no other blocks in the region

Problem/Motivation

The PlaceBlock config action (placeBlockInDefaultTheme, placeBlockInAdminTheme) tries to ensure that the placed block has the correct weight. The problem is that, if there are no other blocks in the region, it tries to call methods on FALSE, which is a fatal error.

We should ensure that it can safely place blocks into an empty region.