Add empty string normalization to sqllogictests

[alamb]

Which issue does this PR close?

Part of apache/arrow-rs#3268

Rationale for this change

Follow the DuckDB semantics (so we can eventually reuse some/all of their tests) https://duckdb.org/dev/sqllogictest/result_verification#null-values-and-empty-strings

What changes are included in this PR?

Normalize empty strings to (empty)

Note that I can't normalize nulls to NULL without apache/arrow-rs#3268

Are these changes tested?

Yes

Are there any user-facing changes?

No

[alamb]

fyi @xudong963 and @mvanschellebeeck

[xudong963]

👍

[ursabot]

Benchmark runs are scheduled for baseline = b5b50ba and contender = a2bc499. a2bc499 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java