refactor: tweak the horrible code in SearchableField

[larseggert]

So another corner case works.

Fixes #3940. Hopefully doesn't break anything.

[codecov]

Codecov Report

Merging #3957 (95363ba) into main (cb996c5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 95363ba differs from pull request most recent head 12c9580. Consider uploading reports for the commit 12c9580 to get more accurate results

@@           Coverage Diff           @@
##             main    #3957   +/-   ##
=======================================
  Coverage   88.14%   88.14%           
=======================================
  Files         297      297           
  Lines       39273    39273           
=======================================
+ Hits        34618    34619    +1     
+ Misses       4655     4654    -1     

Impacted Files	Coverage Δ
ietf/utils/fields.py	91.32% <100.00%> (ø)
ietf/utils/pipe.py	82.60% <0.00%> (-4.35%)	⬇️
ietf/group/utils.py	94.66% <0.00%> (+0.97%)	⬆️

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