[2.6] CS Fixes: lowercase null constants

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | ?
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

Addition for #12872
We should do the same for TRUE and FALSE, but frankly, we don't care as we are talking about generated code.