GH-48664: [R] Implement support for keepNA = FALSE in base::nchar()

[HyukjinKwon]

Rationale for this change

Should address TODO added in commit 81e1fbc (ARROW-17665)

arrow/r/R/dplyr-funcs-string.R

Line 488 in 744f0ec

# TODO: I think there is a fill_null kernel we could use, set null to 2

Implemented the argument intended to work.

What changes are included in this PR?

This PR Implement support for keepNA = FALSE in base::nchar()

Are these changes tested?

Unittest was added.

Are there any user-facing changes?

Yes. Now nchar(..., keepNA = FALSE) should work.

• GitHub Issue: [R] Implement support for keepNA = FALSE in base::nchar() #48664

[github-actions]

⚠️ GitHub issue #48664 has been automatically assigned in GitHub to PR creator.

[HyukjinKwon]

@jonkeane do you mind if I ask to take a look when you find some time please 🙇

[jonkeane]

@github-actions crossbow submit test-r-arrow-backwards-compatibility

[github-actions]

Revision: 7cc044b

Submitted crossbow builds: ursacomputing/crossbow @ actions-b70471dfc8

Task	Status
test-r-arrow-backwards-compatibility

[jonkeane]

Thanks for this. Would you mind adding a news entry about this since it's a new feature (+ feel free to add your name since you're a relatively new contributor and I don't think you yet have a news entry like this?)

@jonkeane do you mind if I ask to take a look when you find some time please 🙇

Sorry for the delay and thank you for the reminder ping

[HyukjinKwon]

No problem! Thank you so much for your time 🫡 .

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 3 benchmarking runs that have been run so far on merge-commit 435d8ba.

There weren't enough matching historic benchmark results to make a call on whether there were regressions.

The full Conbench report has more details.