fix(eslint-plugin): [dot-notation] handle missing declarations

[bradzacher]

also minor perf improvement

Fixes #1945

[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 #1947 into master will decrease coverage by 0.01%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master    #1947      +/-   ##
==========================================
- Coverage   94.36%   94.35%   -0.02%     
==========================================
  Files         168      168              
  Lines        7664     7664              
  Branches     2204     2204              
==========================================
- Hits         7232     7231       -1     
  Misses        183      183              
- Partials      249      250       +1     

Flag	Coverage Δ
#unittest	94.35% <60.00%> (-0.02%)	⬇️

Impacted Files	Coverage Δ
packages/eslint-plugin/src/rules/dot-notation.ts	86.66% <60.00%> (-6.67%)	⬇️