Fix unsanitized output of scan with strings

[davidwendt]

Description

Fixes output from cudf::scan to return sanitized null entries. The null rows are expected to contain no characters if the input null rows are also empty.
The code logic is changed to compute the output validity mask first and use the mask to help build the gather map used for creating the output strings column. This rework also cleans up an unused parameter from internal functions detail::scan_inclusive and detail::scan_exclusive.
An additional check is added to the scan_test utility to verify the results contain empty null entries.

Closes #13449

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[GregoryKimball]

Thank you @davidwendt!! That was fast!

[davidwendt]

@razajafri Do you want to test this fixes your test from #13449 before I merge it?

[razajafri]

@razajafri Do you want to test this fixes your test from #13449 before I merge it?

Ideally, I would like to do that but I won't be able to get to it until EOD today.

[revans2]

@razajafri Do you want to test this fixes your test from #13449 before I merge it?

Ideally, I would like to do that but I won't be able to get to it until EOD today.

I did a quick run and it is looking good. I would like to run all of our tests, but the ones that were failing before are passing with this change now.

[davidwendt]

/merge

[razajafri]

@davidwendt Thank you for the quick resolution. I just tested this with the plugin and it works