test: add undefined header value test case

[codecov[bot]]

Codecov Report

Merging #442 (6d083da) into master (f40ce0a) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #442   +/-   ##
=======================================
  Coverage   99.51%   99.51%           
=======================================
  Files           8        8           
  Lines        1229     1229           
  Branches      236      236           
=======================================
  Hits         1223     1223           
  Misses          6        6           

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[github-actions[bot]]

:tada: This PR is included in version 3.13.2 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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