release-24.3: sql: fix early exit from merge index planner

[blathers-crl]

Backport 1/1 commits from #155261 on behalf of @mw5h.

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The MergeIndexes() method of the IndexBackfillerMergePlanner has a check to see if there are any spans to merge before proceeding with building the plan for the merge. Previously the check for this was checking whether there were any elements in spansToDo, which is allocated to have one entry per source index ID, causing the length check to be vacuously true. The logic probably meant to to check the length of the spans list for each individual source index ID, which this patch puts into place.

Epic: none
Release note: None

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Release justification:

[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 #155266 does not comply with backport policy

Confidence: high
Explanation: The PR involves changes to a production file (pkg/sql/mvcc_backfiller.go) and lacks comprehensive justification for backporting within the PR body. As per the policy, changes to production files in a backport must fix a critical bug or be clearly justified as critically necessary and approved. Though it appears to address a logic error that could affect the execution correctness, the PR does not explicitly substantiate this fix as a critical bug based on the provided criteria (stability, data loss, performance regression, incorrect results, or debugging production issues). Additionally, there is no explicit 'Release justification:' provided in the PR body that elucidates why this change is critically necessary; thus, it fails to meet the exceptions outlined in the backport policy. A detailed review of the code change suggests a logic fix but without an explicit context or demonstration of critical impact. Furthermore, there is no mention of a feature flag, nor does it meet any requirements that deem it backward compatible as it might alter existing functionality.
Recommendation: Request a detailed 'Release justification' explaining why this is a critical bug that requires immediate backporting or reconsider the backport.

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

[cockroach-teamcity]

This change is