sql: fix relocate commands with NULLs

[yuzefovich]

Previously, we would crash when evaluating EXPERIMENTAL_RELOCATE commands when some of the values involved where NULL, and this is now fixed. There is no release note since the commands are "experimental" after all.

Fixes: #87371.

Release note: None

[cockroach-teamcity]

This change is 

[mgartner]

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

[msirek]

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

[yuzefovich]

TFTRs!

bors r+

[craig]

Build failed (retrying...):

• Bazel Essential CI (Cockroach)

[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 creating merge commit from e2be3aa to blathers/backport-release-21.2-88182: 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 21.2.x failed. See errors above.

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error creating merge commit from e2be3aa to blathers/backport-release-22.1-88182: 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 22.1.x failed. See errors above.

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