[Bug] NullPointerException when renaming a method using setSimpleName on a VirtualFile

Hi everyone,

I am unsure whether #3744 introduced a regression when calling `setSimpleName` on a model built from a `VirtualFile`. Is it valid to combine a `VirtualFile` and the `SniperJavaPrettyPrinter`?

Here is a minimal example that will throw a `NullPointerException`:

```java
@Test
public void reproduction() {
var resource = new VirtualFile("public class SomeClass { void SomeMethod() {}}");
Launcher launcher = new Launcher();
launcher.addInputResource(resource);
launcher.getEnvironment().setPrettyPrinterCreator(
() -> new spoon.support.sniper.SniperJavaPrettyPrinter(launcher.getEnvironment())
);
launcher.buildModel();
var model = launcher.getModel();
var executables = model.getElements(new TypeFilter<>(CtExecutable.class));
var element = executables.get(1);
element.setSimpleName("SomeOtherMethod");
}
```

Exception location:
https://github.com/INRIA/spoon/pull/3744/files#diff-befbcc64ff160d86d9fa914487968edee1031311321a89dafbae1d0f69e4ee3bR254

`spoon.support.sniper.internal.ElementSourceFragment.fromSameFile` as the `SourcePosition.getFile()` call returns null.