Add ability to inject null values

[IvanGoncharov]

Suggested by @williamcabrera4 here: https://github.com/williamcabrera4/graphql-faker/commit/ddb748b77844c881a7a14a4abc1b42a01f1fb470#commitcomment-22120829

[niklasR]

This would be pretty cool - we've had issues where clients have expected content in nullable fields. It'd be useful to be able to randomly have nullable fields nulled throughout the whole response.

[vasilich6107]

Any new on this issue?