ARROW-18305: [R] Fix for dev purrr

[hadley]

The recycling rules in map2() are now stricter, so we need to check that x actually has columns before applying the metadata.

I also mildly refactored the test to make it easier to run in isolation; I'm happy to revert those changes if desired.

[nealrichardson]

Thanks!

[ursabot]

Benchmark runs are scheduled for baseline = 8074496 and contender = 04917f9. 04917f9 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 ⬇️2.91% ⬆️1.68%] test-mac-arm
[Finished ⬇️0.0% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️1.14% ⬆️0.11%] ursa-thinkcentre-m75q
Buildkite builds:
[Finished] 04917f94 ec2-t3-xlarge-us-east-2
[Finished] 04917f94 test-mac-arm
[Finished] 04917f94 ursa-i9-9960x
[Finished] 04917f94 ursa-thinkcentre-m75q
[Finished] 8074496c ec2-t3-xlarge-us-east-2
[Finished] 8074496c test-mac-arm
[Finished] 8074496c ursa-i9-9960x
[Finished] 8074496c 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

[github-actions]

https://issues.apache.org/jira/browse/ARROW-18305