haxe 4.1.4 - inconsistent behaviour of sys.FileSystem.fullPath

Haxe Compiler 4.1.4

`sys.FileSystem.fullPath` works different.

`TestFullPath.hx`
```
class TestFullPath {
public static function main() {
trace(sys.FileSystem.fullPath("./not_existence_path")); //
}
}
```

```haxe --run TestFullPath```

output on windows os:
```
TestFullPath.hx:3: D:\devel\haxelib-dev\nme\samples\00-Nmml\not_existence_path
```
on linux os it leads to crash:
```
Uncaught exception Failure("get_full_path")
```

Start with the TestFullPath.hx reproduction and run `haxe --run TestFullPath` on Windows and Linux. Compare how sys.FileSystem.fullPath handles the nonexistent relative path; done means the call no longer crashes on Linux and its behavior is consistent across the two platforms.

Written by the indexing model from the issue text.