Fix crash when optimizing rotation

probably helps to solve #8319

src/slic3r/GUI/Jobs/RotoptimizeJob.cpp

@@ -102,6 +102,7 @@ void RotoptimizeJob::finalize(bool canceled, std::exception_ptr &eptr)
             auto    trmatrix = oi->get_transformation().get_matrix();
             Polygon trchull  = o->convex_hull_2d(trmatrix);
             
+            if (!trchull.empty()) {
                 MinAreaBoundigBox rotbb(trchull, MinAreaBoundigBox::pcConvex);
                 double            phi = rotbb.angle_to_X();
     
@@ -112,6 +113,7 @@ void RotoptimizeJob::finalize(bool canceled, std::exception_ptr &eptr)
     
                 oi->set_rotation(rt);
             }
+        }
 
         // Correct the z offset of the object which was corrupted be
         // the rotation