Average profile visualization: empty profile is displayed when the target rectangular area is outside the image area

[PierreRaybaut]

When using the "Average profile" feature on an image, a non-empty profile is displayed when the target rectangular area is outside the image area (e.g. when the image is zoomed out and the target area is outside the image area).

This is not expected behavior, as the profile should be empty in this case.

This is caused by the fact that the get_closest_index_rect method of image items is avoiding to return an empty rectangular area as it should when the target area is outside the image area.