sql: fix nil pointer dereference in crdb_internal.transaction_contention_events

[zachlite]

schemaDesc was not checked for a nil pointer before dereferencing it. schemaDesc can be nil under the following conditions:

1. A schema descriptor can be deleted, in which case all of its tables would be deleted too.

2. The transaction used to retrieve the descriptor fails.

Given the structure of getContentionEventInfo, this nil pointer dereference is only possible under scenario 2: If a schema and its tables are deleted, a nil table descriptor would force an early return, and this code path would not execute.

Fixes: #104406
Epic: none
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?

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[cockroach-teamcity]

This change is 

[maryliag]

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @yuzefovich)

[yuzefovich]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! 2 of 0 LGTMs obtained (waiting on @zachlite)

[zachlite]

bors r+

[zachlite]

TFTRs!

[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 setting reviewers, but backport branch blathers/backport-release-23.1-106185 is ready: POST https://api.github.com/repos/cockroachdb/cockroach/pulls/106210/requested_reviewers: 422 Reviews may only be requested from collaborators. One or more of the teams you specified is not a collaborator of the cockroachdb/cockroach repository. []

Backport to branch 23.1.x failed. See errors above.

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