Enable nullable context for the tests

the test project doesnt have "nullable" context enabled. So, I couldnt do the suggested. Enabling nullable context for test project will be a refactoring issue in itself.

It errors:

> Severity Code Description Project File Line Suppression State
> Error CS8632 The annotation for nullable reference types should only be used in code within a '#nullable' annotations context. 1576 Active

_Originally posted by @Aniruddh25 in https://github.com/Azure/data-api-builder/pull/1823#discussion_r1367129556_