Use - in addition to the empty string

[AlekSi]

Description

Closes #3699.

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

Merging #3704 (110607e) into main (db7342a) will decrease coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3704      +/-   ##
==========================================
- Coverage   76.36%   76.23%   -0.13%     
==========================================
  Files         323      323              
  Lines       23029    23029              
==========================================
- Hits        17585    17557      -28     
- Misses       4406     4434      +28     
  Partials     1038     1038              

see 6 files with indirect coverage changes

Flag	Coverage Δ
filter-true	72.27% <ø> (-0.13%)	⬇️
hana-1	?
integration	72.27% <ø> (-0.13%)	⬇️
mongodb-1	5.47% <ø> (ø)
postgresql-1	52.43% <ø> (-0.06%)	⬇️
postgresql-2	51.63% <ø> (-0.01%)	⬇️
postgresql-3	50.69% <ø> (ø)
sort-false	72.27% <ø> (-0.13%)	⬇️
sqlite-1	51.83% <ø> (ø)
sqlite-2	50.85% <ø> (-0.01%)	⬇️
sqlite-3	49.90% <ø> (ø)
unit	30.08% <ø> (ø)

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