fix: transfer nullish coalescing assignment(??=) syntax(#2117)

[quanru]

close #2117

[KhafraDev]

Can you add a test? These shouldn't be exported in node 14.

[quanru]

Node 14 is EOL. We have dropped support.

I found engines described in package.json is ">= 14": https://github.com/nodejs/undici/blob/9d30456aa6f195b83ea4ba36ed0b51a951e6bd87/package.json#L107

I got it #2119.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.26 :warning:

Comparison is base (9d30456) 85.86% compared to head (eb3a810) 85.61%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2118      +/-   ##
==========================================
- Coverage   85.86%   85.61%   -0.26%     
==========================================
  Files          76       75       -1     
  Lines        6610     6609       -1     
==========================================
- Hits         5676     5658      -18     
- Misses        934      951      +17     

Impacted Files	Coverage Δ
lib/cookies/parse.js	90.90% <100.00%> (ø)
lib/fetch/headers.js	97.50% <100.00%> (ø)
lib/websocket/receiver.js	84.51% <100.00%> (ø)

... and 4 files with indirect coverage changes

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[Uzlopak]

@ronag We can close this as engines in package.json is now set to >= 18.