sql: fix current_setting(..., true) for custom options

[rafiss]

Release note (bug fix): The current_setting builtin function now properly does not result in an error when checking a custom session setting that does not exist and the missing_ok argument is true.

[cockroach-teamcity]

This change is 

[ZhouXing19]

LGTM, thanks!

[rafiss]

tftr!

bors r=ZhouXing19

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from 888bf2b to blathers/backport-release-21.2-88139: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 21.2.x failed. See errors above.

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error creating merge commit from 888bf2b to blathers/backport-release-22.1-88139: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 22.1.x failed. See errors above.

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