fix: broken serve with new CLI API

[alexander-akait]

What kind of change does this PR introduce?

bugfix, my mistake, check will be bad for webpack v4 and webpack-dev-server the next version

Did you add tests for your changes?

Latest, but we will have these test in webpack-dev-server

If relevant, did you update the documentation?

No need

Summary

Fix new CLI API

Does this PR introduce a breaking change?

No

Other information

I will do the new patch release after merge

[codecov]

Codecov Report

Merging #2770 (8053b5e) into master (598008a) will decrease coverage by 0.05%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2770      +/-   ##
==========================================
- Coverage   94.16%   94.10%   -0.06%     
==========================================
  Files          31       31              
  Lines        1679     1680       +1     
  Branches      488      489       +1     
==========================================
  Hits         1581     1581              
- Misses         98       99       +1     

Impacted Files	Coverage Δ
packages/serve/src/index.ts	64.48% <0.00%> (-0.61%)	⬇️

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