Fix thicken normal calculation when no override is provided

[danieledapo]

Do not calculate normals if the user did not provide any override, it makes the calculation coherent with what OCCT does.

Fixes #623.

[gumyr]

The real problem here is that the normal_override parameter is being changed for subsequent faces. Here is a smaller change that just renames the variable:

    for face in to_thicken_faces:
        face_normal = (
            normal_override if normal_override is not None else face.normal_at()
        )
        for direction in [1, -1] if both else [1]:
            new_solids.append(
                face.thicken(depth=amount, normal_override=face_normal * direction)
            )

I'd like to go this this less complex change.

[danieledapo]

good catch! should be ok now!

[gumyr]

Thanks!