Global shader sampler variables don't store their values

### Tested versions

Tested in: Godot v4.3.dev [40b4130c9] and v4.3.dev6.official [89850d553]

### System information

Godot v4.3.dev (40b4130c9) - Windows 10.0.19045 - Vulkan (Forward+) - dedicated AMD Radeon RX 580 2048SP (Advanced Micro Devices, Inc.; 31.0.21912.14) - AMD Ryzen 5 3600 6-Core Processor (12 Threads)

### Issue description

Godot don't store the data from any sampler variable type, after set the value in project settings restarting the editor make all the sampler variables go back to empty, the exception is if you use a resource already stored in your project directory, in this case the value will be preserved after the restart

https://github.com/godotengine/godot/assets/44306054/ae47ecba-60c8-42e5-b6ea-ff5119c74986

### Steps to reproduce

Create a new global shader variable in project settings of any sampler type
Create a new resource inside the variable
Close project settings and restart the editor

When you open the global shaders again the variable will be empty

### Minimal reproduction project (MRP)

No needed, any project can reproduce that.

Reproduce the issue from Project Settings by creating a global shader variable with a sampler resource, closing the settings, and restarting the editor. Verify whether the sampler value is empty afterward, then trace the global shader variable persistence path and add coverage for sampler resources surviving an editor restart.

Written by the indexing model from the issue text.