Add null-equals-null join support

[nseekhao]

Which issue does this PR close?

Closes #5084

Rationale for this change

Support for null-equals-null joins as described in #5084.

What changes are included in this PR?

• Support null-equals-null joins in both producer and consumer
• Support for accepting ScalarValue::UInt8 in the producer. (Encoded as i32 since Substrait does not support unsigned integers)

Are these changes tested?

Yes

Are there any user-facing changes?

No

[ursabot]

Benchmark runs are scheduled for baseline = a780a0b and contender = ad3cc24. ad3cc24 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