GH-15109: [Python] Allow creation of non empty struct array with zero field

[0x26res]

@jorisvandenbossche

• I made minor changes to the C++ code, I couldn't help myself
• I've followed your suggestion, I've used {} to represent an empty struct python value, instead of []. Because that's what is used by as_py
• I've also passed the mask to pa.array because we want it to be applied to the resulting struct array.

• Closes: [Python] Can't create a non empty StructArray with no field using StructArray.from_array #15109

[github-actions]

• Closes: [Python] Can't create a non empty StructArray with no field using StructArray.from_array #15109

[github-actions]

⚠️ GitHub issue #15109 has been automatically assigned in GitHub to PR creator.

[jorisvandenbossche]

Looks good to me, thanks!

[ursabot]

Benchmark runs are scheduled for baseline = e920474 and contender = fc1f9eb. fc1f9eb 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 ⬇️1.11% ⬆️0.0%] test-mac-arm
[Failed] ursa-i9-9960x
[Failed] ursa-thinkcentre-m75q
Buildkite builds:
[Finished] fc1f9ebb ec2-t3-xlarge-us-east-2
[Finished] fc1f9ebb test-mac-arm
[Failed] fc1f9ebb ursa-i9-9960x
[Failed] fc1f9ebb ursa-thinkcentre-m75q
[Finished] e920474d ec2-t3-xlarge-us-east-2
[Finished] e920474d test-mac-arm
[Failed] e920474d ursa-i9-9960x
[Finished] e920474d 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

[ursabot]

['Python', 'R'] benchmarks have high level of regressions.
test-mac-arm