Add support for serializing null scalar values

[andygrove]

Which issue does this PR close?

Closes #3302

Rationale for this change

Needed to support certain SQL queries.

What changes are included in this PR?

Add serde support for untyped NULL literal

Are there any user-facing changes?

No

[andygrove]

@carols10cents Could you take a look if you have time since you worked on this code before?

[andygrove]

@yahoNanJing @thinkharderdev @avantgardnerio This issue came up when testing with Ballista, so you may want to review this as well

[codecov-commenter]

Codecov Report

Merging #3303 (ed4ced7) into master (a32c408) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3303   +/-   ##
=======================================
  Coverage   85.74%   85.75%           
=======================================
  Files         294      294           
  Lines       53736    53747   +11     
=======================================
+ Hits        46076    46089   +13     
+ Misses       7660     7658    -2     

Impacted Files	Coverage Δ
datafusion/proto/src/from_proto.rs	35.13% <100.00%> (+0.11%)	⬆️
datafusion/proto/src/lib.rs	93.52% <100.00%> (+0.11%)	⬆️
datafusion/proto/src/to_proto.rs	51.07% <100.00%> (+0.08%)	⬆️
datafusion/common/src/scalar.rs	84.58% <0.00%> (-0.07%)	⬇️
datafusion/expr/src/logical_plan/plan.rs	78.55% <0.00%> (+0.17%)	⬆️
datafusion/core/src/physical_plan/metrics/value.rs	87.43% <0.00%> (+0.50%)	⬆️

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[alamb]

LGTM -- thanks @andygrove

[ursabot]

Benchmark runs are scheduled for baseline = fcd2275 and contender = 70322e5. 70322e5 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