populate empty resonse for IndexStatsResponse and VolumeResponse

[owen-d]

Fixes a panic when the underlying response struct is accessed but not populated after an out-of-bounds request is short-circuited via NewEmptyResponse here. This PR ensures the embedded types are not nil in accordance with the other variants in this function.

[jeschkies]

Nice one. Could you extend the tests?

[grafanabot]

The backport to k174 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-11209-to-k174 origin/k174
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 265018b7293329241ffe9b7c56f38afb07b6b08f

When the conflicts are resolved, stage and commit the changes:

git add . && git cherry-pick --continue

If you have the GitHub CLI installed:

# Push the branch to GitHub:
git push --set-upstream origin backport-11209-to-k174
# Create the PR body template
PR_BODY=$(gh pr view 11209 --json body --template 'Backport 265018b7293329241ffe9b7c56f38afb07b6b08f from #11209{{ "\n\n---\n\n" }}{{ index . "body" }}')
# Create the PR on GitHub
echo "${PR_BODY}" | gh pr create --title "[k174] populate empty resonse for IndexStatsResponse and VolumeResponse" --body-file - --label "size/S" --label "type/bug" --label "backport" --base k174 --milestone k174 --web

Or, if you don't have the GitHub CLI installed (we recommend you install it!):

# Push the branch to GitHub:
git push --set-upstream origin backport-11209-to-k174

# Create a pull request where the `base` branch is `k174` and the `compare`/`head` branch is `backport-11209-to-k174`.

# Remove the local backport branch
git switch main
git branch -D backport-11209-to-k174