GH-14855: [C++] Support importing zero-case unions

[lidavidm]

Closes #14855.

• Closes: [C++] Zero-case union can't be imported via C Data Interface #14855

[github-actions]

• Closes: [C++] Zero-case union can't be imported via C Data Interface #14855

[pitrou]

Thanks! In addition to the schema tests, can you add similar tests for array import/export (or roundtrip)?

[lidavidm]

Done.

[pitrou]

Thanks a lot @lidavidm !

[ursabot]

Benchmark runs are scheduled for baseline = 45d83fe and contender = 34672cb. 34672cb is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Finished ⬇️0.0% ⬆️0.0%] ec2-t3-xlarge-us-east-2
[Finished ⬇️0.0% ⬆️0.0%] test-mac-arm
[Finished ⬇️0.0% ⬆️0.0% ⚠️ Contender and baseline run contexts do not match] ursa-i9-9960x
[Finished ⬇️0.0% ⬆️0.0% ⚠️ Contender and baseline run contexts do not match] ursa-thinkcentre-m75q
Buildkite builds:
[Finished] 34672cb2 ec2-t3-xlarge-us-east-2
[Finished] 34672cb2 test-mac-arm
[Finished] 34672cb2 ursa-i9-9960x
[Finished] 34672cb2 ursa-thinkcentre-m75q
[Finished] 45d83fec ec2-t3-xlarge-us-east-2
[Finished] 45d83fec test-mac-arm
[Finished] 45d83fec ursa-i9-9960x
[Finished] 45d83fec ursa-thinkcentre-m75q
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