[SQLite] introduce Nullable entity

Summary:
We need to introduce something that emulates pointers in C++ because of Emscripten limitations.

Context here: ENG-5719

Test Plan:

1. In C++ tested by creating objects and calling method/using operator.
2. In Emscripten tested later in the stack.

Reviewers: tomek, marcin, michal

Reviewed By: marcin

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9902