refactor: use optional chaining & delete unnecessary assertion

[noritaka1166]

What kind of change does this PR introduce?
refactoring

Did you add tests for your changes?
no

If relevant, did you update the documentation?
no

Summary
use optional chaining & delete unnecessary assertion

Does this PR introduce a breaking change?
no

Other information
no

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.94%. Comparing base (31f634a) to head (79a8c65).
Report is 239 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4465      +/-   ##
==========================================
- Coverage   90.18%   89.94%   -0.25%     
==========================================
  Files          22        9      -13     
  Lines        1702     1412     -290     
  Branches      491      461      -30     
==========================================
- Hits         1535     1270     -265     
+ Misses        167      142      -25     

Files with missing lines	Coverage Δ
packages/webpack-cli/src/webpack-cli.ts	91.04% <100.00%> (-1.60%)	⬇️

... and 3 files with indirect coverage changes

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[alexander-akait]

Thanks