release-25.4: sql: add nil check for extendedEvalContext.Placeholders

[blathers-crl]

Backport 1/1 commits from #161908 on behalf of @ZhouXing19.

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Fixes #161674

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

Release note: None

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Release justification: prevent a nil pointer crash in an edge case.

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that it falls into one of the following categories (select one):

• Non-production code changes. Includes test-only changes, build system changes, etc.
• Fixes for serious issues. Defined in the policy as correctness, stability, or security issues, data corruption/loss, significant performance regressions, breaking working and widely used functionality, or an inability to detect and debug production issues.
• Other approved changes. These changes must be gated behind a disabled-by-default feature flag unless there is a strong justification not to.

Add a brief release justification to the PR description explaining your selection.

Also, confirm that the change does not break backward compatibility and complies with all aspects of the backport policy.

All backports must be reviewed by the TL and EM for the owning area.

[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.}

[blathers-crl]

❌ PR #161911 does not comply with backport policy

Confidence: medium
Explanation: This PR is aimed at preventing a nil pointer crash by adding a nil check in the pkg/sql/event_log.go file. According to the CockroachDB Backport Policy, the PR must address a critical bug or be gated behind a feature flag that is disabled by default. While preventing a nil pointer error can be categorized as addressing a stability issue (a critical bug), there is no clear evidence or confirmation that the issue meets the outlined 'critical bug' criteria such as causing data corruption, significant performance regression, or suboptimal performance. Moreover, there is no feature flag associated with this change.
Recommendation: Reconsider the backport or provide more evidence that the bug should be considered critical enough to warrant a backport without the need for a feature flag.

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

[cockroach-teamcity]

This change is 

[github-actions]

Reminder: it has been 2 weeks please merge or close your backport!

[yuzefovich]

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

[mw5h]

This is fine to backport, but you need a release justification.