`AtlasTexture` margin size.x/y has no effect if corresponding region size.x/y is zero

### Tested versions

4.4 beta 2

### System information

Windows 10

### Issue description

Docs say:
> If either dimension of the region's size is 0, the value from atlas size will be used for that axis instead

But it does not work well with margins as issue title says:

| One axis is zero | Both axises are zero |
| --- | --- |
|  |  |

Discovered during https://github.com/godotengine/godot/pull/102467

This behaviour may be confusing. Worth to fix in code or document it.

### Steps to reproduce

Add archive content from "MRP" to project, set region size to (0, 0), try to change margin size.

### Minimal reproduction project (MRP)

[atlas_texture_bugged.zip](https://github.com/user-attachments/files/18683785/atlas_texture_bugged.zip)