fix(eslint-plugin): [no-unnecessary-condition] better handling for unary negation

[renugasaraswathy]

Fixes #2255

Added handing for unary negation expression.
cc @bradzacher

Please let me know if any change is required.

[typescript-eslint]

Thanks for the PR, @renugasaraswathy!

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 #2382 into master will decrease coverage by 0.01%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master    #2382      +/-   ##
==========================================
- Coverage   93.16%   93.15%   -0.02%     
==========================================
  Files         286      286              
  Lines        9161     9174      +13     
  Branches     2517     2524       +7     
==========================================
+ Hits         8535     8546      +11     
  Misses        301      301              
- Partials      325      327       +2     

Flag	Coverage Δ
#unittest	93.15% <84.61%> (-0.02%)	⬇️

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	96.02% <84.61%> (-0.91%)	⬇️

[renugasaraswathy]

Will work on the code coverage and push.

[bradzacher]

thanks!