Fix T77105: Crashes when clicking "new scene" button

This was introduced on ecc395e473.

Effectively this is reverting that commit for cases when
scene->toolsettings->sculpt is NULL. But since the facesets are only
working for sculpting this should be fine.

source/blender/draw/engines/overlay/overlay_engine.c

@@ -78,12 +78,15 @@ static void OVERLAY_engine_init(void *vedata)
   if (v3d->shading.type == OB_WIRE) {
     pd->overlay.flag |= V3D_OVERLAY_WIREFRAMES;
   }
+
+  if (ts->sculpt) {
     if (ts->sculpt->flags & SCULPT_HIDE_FACE_SETS) {
       pd->overlay.sculpt_mode_face_sets_opacity = 0.0f;
     }
     if (ts->sculpt->flags & SCULPT_HIDE_MASK) {
       pd->overlay.sculpt_mode_mask_opacity = 0.0f;
     }
+  }
 
   pd->use_in_front = (v3d->shading.type <= OB_SOLID) ||
                      BKE_scene_uses_blender_workbench(draw_ctx->scene);