### Godot version

Godot Engine v4.0.stable.official.92bee43ad

### System information

Windows 10

### Issue description

The documentation states that passing ```null``` into the image parameter of ```set_custom_mouse_cursor``` will change it back to the system cursor, however I find that it removes the cursor's image entirely, rather than setting it to the system's cursor.


### Steps to reproduce

```var isimage = false
var theimage = load("res://final.png")

func _on_my_button_pressed():
    if isimage == false:
        Input.set_custom_mouse_cursor(theimage, Input.CURSOR_ARROW, Vector2(16,16))
        isimage = true
        
    else:
        Input.set_custom_mouse_cursor(null)
        isimage = false```

When the button is pressed, the cursor is meant to change to the new cursor. When it has been pressed a second time, it changes back to the default cursor. However, upon pressing it a second time, it simply disappears. The cursor becomes visible again when the cursor exits and re-enters the window.

### Minimal reproduction project

[null mouse.zip](https://github.com/godotengine/godot/files/10901969/null.mouse.zip)

"fleur",
			"dnd-move",
			"crossed_circle",
			"v_double_arrow",
			"h_double_arrow",
			"size_bdiag",
			"size_fdiag",
			"move",
			"row_resize",
			"col_resize",
			"question_arrow"
		};
		cursor_img[i] = XcursorLibraryLoadImage(cursor_file[i], cursor_theme, cursor_size);
		if (!cursor_img[i]) {
			const char *fallback = nullptr;
			switch (i) {
				case CURSOR_POINTING_HAND:
					fallback = "pointer";
					break;
				case CURSOR_CROSS:
					fallback = "crosshair";
					break;
				case CURSOR_WAIT:
					fallback = "wait";
					break;
				case CURSOR_BUSY:
					fallback = "progress";
					break;
				case CURSOR_DRAG:
					fallback = "grabbing";