Fix embedded package panic

[noisersup]

Description

Closes #4235.

Readiness checklist

• I added/updated unit tests (and they pass).
• I added/updated integration/compatibility tests (and they pass).
• I added/updated comments and checked rendering.
• I made spot refactorings.
• I updated user documentation.
• I ran task all, and it passed.
• I ensured that PR title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Milestone (Next), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.21%. Comparing base (2665671) to head (90d9854).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4278      +/-   ##
==========================================
- Coverage   75.27%   74.21%   -1.06%     
==========================================
  Files         323      323              
  Lines       22355    22356       +1     
==========================================
- Hits        16827    16591     -236     
- Misses       4305     4561     +256     
+ Partials     1223     1204      -19     

Files	Coverage Δ
ferretdb/ferretdb.go	86.66% <100.00%> (+0.14%)	⬆️

... and 29 files with indirect coverage changes

Flag	Coverage Δ
filter-false	?
filter-true	67.49% <ø> (-1.15%)	⬇️
hana-1	?
integration	67.49% <ø> (-1.18%)	⬇️
mongodb-1	5.14% <ø> (+0.01%)	⬆️
mysql-1	?
mysql-2	?
mysql-3	?
postgresql-1	44.21% <ø> (-9.37%)	⬇️
postgresql-2	43.66% <ø> (-10.21%)	⬇️
postgresql-3	42.32% <ø> (-11.26%)	⬇️
postgresql-4	40.40% <ø> (-0.86%)	⬇️
postgresql-5	46.17% <ø> (+2.82%)	⬆️
sqlite-1	43.47% <ø> (-9.07%)	⬇️
sqlite-2	42.92% <ø> (-9.95%)	⬇️
sqlite-3	41.74% <ø> (-10.98%)	⬇️
sqlite-4	39.66% <ø> (-0.96%)	⬇️
sqlite-5	45.22% <ø> (+2.75%)	⬆️
unit	33.29% <100.00%> (+1.33%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

[AlekSi]

We must add tests for fixed bugs reported by users

[rumyantseva]

The change looks good, even better if we can add tests, yes :)

[rumyantseva]

Looks good!