sql: add nil check for extendedEvalContext.Placeholders

[ZhouXing19]

Fixes #161674

We saw a nil pointer error with pausable portal, so adding a nil check gate here.

Release note: None

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[yuzefovich]

Thanks!

@yuzefovich reviewed 1 file and all commit messages, and made 1 comment.
Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @DrewKimball).

[ZhouXing19]

TFTR!
bors r+

[craig]

Build succeeded:

• local_roachtest_fips
• check_generated_code
• docker_image_amd64
• examples_orms
• local_roachtest
• lint
• linux_amd64_build
• linux_amd64_fips_build
• unit_tests
• acceptance

[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.

---

💡 Consider backporting to the fork repo instead of the main repo. See instructions for more details.

error creating merge commit from ae862df to blathers/backport-release-24.3-161908: 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 release-24.3 failed. See errors above.

---

💡 Consider backporting to the fork repo instead of the main repo. See instructions for more details.

error creating merge commit from ae862df to blathers/backport-release-25.2-161908: 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 release-25.2 failed. See errors above.

---

💡 Consider backporting to the fork repo instead of the main repo. See instructions for more details.

error creating merge commit from ae862df to blathers/backport-release-25.3-161908: 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 release-25.3 failed. See errors above.

---

💡 Consider backporting to the fork repo instead of the main repo. See instructions for more details.

💡 Consider backporting to the fork repo instead of the main repo. See instructions for more details.

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}