GDExtension classes are reported as an empty string in the `--verbose` "Leaked instance:" output

### Tested versions

Tested in 4.6. Also happens in previous versions. Definitely not a regression.

### System information

Godot v4.6.stable - Debian GNU/Linux forky/sid forky on Wayland - X11 display driver, Single-window, 1 monitor - OpenGL 3 (Compatibility) - Mesa Intel(R) Iris(R) Xe Graphics (RPL-P) - 13th Gen Intel(R) Core(TM) i7-1360P (16 threads) - 15.31 GiB memory

### Issue description

When a gdextension class is leaked and you use `--verbose` to see what was leaked, instead of a class name and reference count, you get something like this, which isn't particularly helpful:
```log
Leaked instance: :9223379987644627892
Leaked instance: :9223379987795622837
Leaked instance: :9223379987745291190
Leaked instance: :9223380438884629431
Leaked instance: :9223380074483497912
Leaked instance: :9223380435713735609
Leaked instance: :9223380437206907834
Leaked instance: :9223379987376192443
Leaked instance: :9223380073476864956
Leaked instance: :9223380443297037245
Leaked instance: :9223380000277871550
Leaked instance: :9223380449034845119
Leaked instance: :9223379987426524096
```

This is because the gdextension is already unloaded by this point, so Godot doesn't know what the classes are named anymore.

### Steps to reproduce

1. Make a gdextension that registers at least one class
2. Leak instances of that class
3. Run the project with `--verbose`

### Minimal reproduction project (MRP)

N/A