fix(typescript-estree): only create project service from env setting if project is enabled

[JoshuaKGoldberg]

PR Checklist

• Addresses an existing open issue: fixes Repo: Our EXPERIMENTAL_useProjectService unit tests are much slower in CI #7348; fixes Repo: Failure on main: Run Unit Tests with Experimental TSServer (eslint-plugin)  #8131
• That issue was marked as accepting prs
• Steps in Contributing were taken

Overview

I spent some time trying to investigate #8131's unit test failures. I made a bit of progress and then realized... wait, why is the project service enabled for either of these rules? We don't normally enable parserOptions.project for un-typed lint rules. Therefore, I don't think we'd want the presence of the environment variable to enable the project service unless options.project was also set.

[typescript-eslint]

Thanks for the PR, @JoshuaKGoldberg!

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The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

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[fisker]

Hello @JoshuaKGoldberg , should we check it's non-empty?

I don't know this project options, asking this only because I use project:[] in prettier and eslint-plugin-unicorn, I don't remember why, maybe prevent search for tsconfig.json?

[fisker]

Looking at the docs, maybe I should use project: false?

[JoshuaKGoldberg]

Hmm, yes, or project: null.

[fisker]

Back to my question, should we treat [] as false here?

[JoshuaKGoldberg]

Good question 🤔. My gut instinct is that it should probably be an error. I'm not sure though... Filed #8210.