add a new env variable EXTENT_NULLABLE_FLG to allow user to customize…

[rzhang152]

… extent data nullable option

[helrond]

Hi @rzhang152 this looks good! I just suggested some small changes to the config name and comments, mostly thinking ahead to the possibility of other configuration around inheritance of particular data attributes.

I've also been mulling whether or not it's necessary to test this particular branch of the logic, but I think it might not be worth the trouble. If you can think of a way to do it easily (like maybe with mocks and then checking the call count of specific functions) that would be great.

Once you've taken a look at those things I'll merge this to a feature branch in this repository, and from there we should be able to run tests and get the code into base.