Allow empty strings pushdown for Tigris

[noisersup]

Description

Closes #1940.

Readiness checklist

• I added/updated unit tests.
• I added/updated integration/compatibility tests.
• 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), Assignee, Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #2110 (d1809bb) into main (b86480f) will increase coverage by 0.05%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2110      +/-   ##
==========================================
+ Coverage   68.53%   68.59%   +0.05%     
==========================================
  Files         362      362              
  Lines       16373    16370       -3     
==========================================
+ Hits        11222    11229       +7     
+ Misses       4138     4128      -10     
  Partials     1013     1013              

Impacted Files	Coverage Δ
internal/handlers/tigris/tigrisdb/query.go	67.60% <ø> (+1.38%)	⬆️
internal/handlers/common/filter.go	84.40% <0.00%> (ø)
internal/handlers/pg/pgdb/database_metadata.go	78.88% <0.00%> (+2.22%)	⬆️
internal/handlers/pg/pgdb/databases.go	72.34% <0.00%> (+6.38%)	⬆️

Flag	Coverage Δ
integration	64.72% <ø> (+<0.01%)	⬆️
mongodb	5.51% <ø> (+<0.01%)	⬆️
pg	53.06% <ø> (+<0.01%)	⬆️
tigris	45.18% <ø> (+<0.01%)	⬆️
unit	27.62% <ø> (+0.18%)	⬆️

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

[noisersup]

❯ task test-integration-tigris
task: [test-integration-tigris] go test -count=1 -timeout=11m -race=true -tags=ferretdb_debug,ferretdb_tigris,ferretdb_hana -shuffle=on -coverpkg=../... -coverprofile=integration-tigris.txt . -target-backend=ferretdb-tigris -target-unix-socket=true -tigris-urls=127.0.0.1:8081,127.0.0.1:8091,127.0.0.1:8092,127.0.0.1:8093,127.0.0.1:8094 -compat-url=mongodb://127.0.0.1:47017/

ok      github.com/FerretDB/FerretDB/integration        248.893s        coverage: 51.7% of statements in ../...

[AlekSi]

Please also update https://github.com/FerretDB/FerretDB/blob/main/build/deps/tigris.Dockerfile to use beta.45 https://hub.docker.com/r/tigrisdata/tigris-local/tags just to be sure

[rumyantseva]

LGTM

[w84thesun]

👍