### Godot version

4.1.dev

### System information

Win11

### Issue description

I load a non-existing GDScript, but it still returns a valid instance instead of null.
```gdscript
@tool
extends EditorScript

func _run():
	var s = ResourceLoader.load("non_exists.gd", "", ResourceLoader.CACHE_MODE_IGNORE)
	print(s)
	assert(s == null)

```

```
  Attempt to open script 'res://non_exists.gd' resulted in error 'File not found'.
<GDScript#-9223369015982753437>
  res://new_script.gd:7 - Assertion failed.

```

### Steps to reproduce

N/A

### Minimal reproduction project

N/A
MutexLock lock(singleton->mutex);
	if (!p_owner.is_empty()) {
		singleton->dependencies[p_owner].insert(p_path);
	}
	Ref<GDScript> script;
	r_error = OK;
	if (singleton->full_gdscript_cache.has(p_path)) {
		script = singleton->full_gdscript_cache[p_path];
		if (!p_update_from_disk) {
			return script;
		}
	}
	if (script.is_null()) {
		script = get_shallow_script(p_path, r_error);
		if (r_error) {
			return script;
		}
	}
	if (p_update_from_disk) {
		r_error = script->load_source_code(p_path);
	}
	if (r_error) {
		return script;
	}
	singleton->full_gdscript_cache[p_path] = script;
	singleton->shallow_gdscript_cache.erase(p_path);
	script->reload(true);
	return script;
}
Ref<GDScript> GDScriptCache::get_cached_script(const String &p_path) {