fix(eslint-plugin): [no-unnecessary-condition] correct regression with unary negations

[bradzacher]

Fixes #2421

[typescript-eslint]

Thanks for the PR, @bradzacher!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

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.

Thanks again!

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[codecov]

Codecov Report

Merging #2422 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2422      +/-   ##
==========================================
+ Coverage   93.12%   93.16%   +0.04%     
==========================================
  Files         287      287              
  Lines        9189     9186       -3     
  Branches     2530     2528       -2     
==========================================
+ Hits         8557     8558       +1     
+ Misses        302      301       -1     
+ Partials      330      327       -3     

Flag	Coverage Δ
#unittest	93.16% <100.00%> (+0.04%)	⬆️

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	98.26% <100.00%> (+2.24%)	⬆️