Finalize `NullShellExecutor`

[aidan-casey]

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The NullShellExecutor is currently tested using mocks. This prevents us from making the class final.

We should revise the tests for that class and finalize the class.

[Qupiter]

Hey there, can you assign this issue to me and open a branch for it? I would like to work with you. This seems like a good issue to start with.

[alibori]

Hi @aidan-casey !

This issue can be closed, isn't it? If I'm right the NullShellExecutor class is final.

Thank you! 😃

[aidan-casey]

Hey, @alibori! It is final, but has no tests. We still need to refactor so we can test it. 🙂

[brendt]

No no, I fixed that, just forgot to close the issue