fix(eslint-plugin): [no-unnecessary-condition] fix false positive with nullish coalescing

[bradzacher]

Fixes #2384

[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 #2385 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2385   +/-   ##
=======================================
  Coverage   93.15%   93.15%           
=======================================
  Files         285      285           
  Lines        9150     9151    +1     
  Branches     2513     2514    +1     
=======================================
+ Hits         8524     8525    +1     
  Misses        301      301           
  Partials      325      325           

Flag	Coverage Δ
#unittest	93.15% <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	96.93% <100.00%> (+0.01%)	⬆️