[v25.1.x] shard_placement_table: unchecked optional fix

[vbotbuildovich]

Backport #28575 to branch v25.1.x. This issue was created as a placeholder because the original PR's commit(s) could not be automatically cherry-picked.
Here are the commands to execute:

git checkout v25.1.x
git checkout -b manual-backport-28575-v25.1.x-713
git cherry-pick -x d958a1ba99

git push origin manual-backport-28575-v25.1.x-713
gh pr create \
  --title "[v25.1.x] shard_placement_table: unchecked optional fix" \
  --base "v25.1.x" \
  --label "kind/backport" \
  --head "manual-backport-28575-v25.1.x-713" \
  --draft \
  --repo "redpanda-data/redpanda" \
  --milestone "v25.1.x-next" \
  --body "Backport of PR https://github.com/redpanda-data/redpanda/pull/28575 "