Tileset Terrain peering to void cells is unreliable

### Tested versions

Using 4.3 stable

### System information

Godot v4.3.stable - macOS 14.6.1 - GLES3 (Compatibility) - Apple M2 Pro - Apple M2 Pro (12 Threads)

### Issue description

When using terrains, I often find that when it has to peer with a void/empty cell, the results are unpredictable / poor.

In the examples below, I will be using https://seliel-the-shaper.itch.io/spring-forest.

Here, you can see it has decided that the water is peered with the "dirt" terrain when it meets the void edge of the map.

As you can see in this, the Deep Water is peered with the Dirt terrain:

Now.. Dirt is the first terrain in the set, so I can kinda see why it might "fall back" to that for a void cell?

To me, it feels like it would be more sensible to preserve the "primary" tile for that terrain; in this case full water.

### Steps to reproduce

Seem to be fairly reliable once you have two+ terrains. The edge seems to always be the "first" terrain?

### Minimal reproduction project (MRP)

I cant really produce an MRP right now with this tilset as its copyright...

There is also a chance it might be a "Meat Based Problem" and I'm using Godot incorrectly...