fix: handle undefined configuration export

[rishabh3112]

What kind of change does this PR introduce?
fix and test

Did you add tests for your changes?
Yup, did in TDD fashion.

If relevant, did you update the documentation?
Not required.

Summary
Treat undefined configuration as a case for zero-config

Does this PR introduce a breaking change?
Nope.

Other information
/cc @alexander-akait

when we have webpack.config.js without module.exports or export we don't work, but no exports means zero configuration and should work

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Rishabh Chawla (eaf9f36, 2328313)

[codecov]

Codecov Report

Merging #2930 (424e0b7) into master (6fc4f79) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2930   +/-   ##
=======================================
  Coverage   95.13%   95.13%           
=======================================
  Files          31       31           
  Lines        1684     1684           
  Branches      481      483    +2     
=======================================
  Hits         1602     1602           
  Misses         82       82           

Impacted Files	Coverage Δ
packages/webpack-cli/lib/webpack-cli.js	96.52% <100.00%> (ø)

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[rishabh3112]

@webpack/cli-team need approvals.

[snitin315]

Few comments.