Fix SPE-2640: Fix overriden extrusions crash

Overriden extrusions must be inserted even if they are empty
because the resulting vector must be the same length as
instances_to_print

src/libslic3r/GCode/ExtrusionOrder.cpp

@@ -421,11 +421,9 @@ std::vector<OverridenExtrusions> get_overriden_extrusions(
             std::vector<SliceExtrusions> slices_extrusions{get_slices_extrusions(
                 print, *layer, should_pick_extrusion, smooth_path, offset, extruder_id, previous_position
             )};
-            if (!slices_extrusions.empty()) {
             result.push_back({offset, std::move(slices_extrusions)});
         }
     }
-    }
     return result;
 }