release-24.2: tree: fix walkStmt for SHOW EXPERIMENTAL_FINGERPRINTS

[blathers-crl]

Backport 1/1 commits from #130895 on behalf of @rafiss.

/cc @cockroachdb/release

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If SHOW EXPERIMENTAL_FINGERPRINTS FROM TABLE appeared in certain kinds of subqueries, the CRDB node could panic while walking the statement due to a missing nil check.

No release note since this only affected an experimental feature.

fixes #130822
Release note: None

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Release justification: bug fix for a panic

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
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• Backports should not add new functionality (except as defined
  here).
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  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
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Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is 

[mgartner]

LGTM