Use optional interval variables with NoOverlap2D constraint for CP model

Currently they [do not work](http://yetanothermathprogrammingconsultant.blogspot.com/2021/02/2d-bin-packing-with-google-or-tools-cp.html) when chained together. Will be supported from CP-SAT v9.3: https://github.com/google/or-tools/issues/3012