fix(eslint-plugin): [no-unnecessary-condition] allow nullish coalescing for naked type parameter

[dora1998]

PR Checklist

• Addresses an existing open issue: fixes Bug: [no-unnecessary-condition] False positive for generic type #6902
• That issue was marked as accepting prs
• Steps in Contributing were taken

Overview

Allow nullish coalescing for a naked type parameter because it might be nullable.

[typescript-eslint]

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[codecov]

Codecov Report

Merging #6910 (a1c766c) into main (40f4eee) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6910   +/-   ##
=======================================
  Coverage   87.36%   87.37%           
=======================================
  Files         386      386           
  Lines       13189    13192    +3     
  Branches     3866     3867    +1     
=======================================
+ Hits        11523    11526    +3     
  Misses       1300     1300           
  Partials      366      366           

Flag	Coverage Δ
unittest	87.37% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...slint-plugin/src/rules/no-unnecessary-condition.ts	98.37% <100.00%> (ø)

... and 1 file with indirect coverage changes

[JoshuaKGoldberg]

LGTM for just the case of a type parameter, thanks! Just requesting a bit more test coverage please. ✨

[JoshuaKGoldberg]

[Testing] The cases of T extends 'a' (always truthy) and T extends 'a' | null (still needing ??) seem to work, but aren't explicitly covered for ?? cases. Could you add in a couple more unit tests along those lines please?

[dora1998]

Oh sorry, I added them!
I also added a alwaysNullish case 😄

[JoshuaKGoldberg]

💯 thanks!