fix: Return empty vector instead of nil for empty evaluator.

[jeschkies]

What this PR does / why we need it:
In case an underlying step evaluator in the query engine would return false for Next() the engine would return nil instead of an empty vector.

This issues has not been noticed because all current evaluators would always return true with an empty vector. However, the new last_over_time and first_over_time merge evaluators broke that convention here. It returns true with an empty vector.

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[grafanabot]

Hello @jeschkies!
Backport pull requests need to be either:

• Pull requests which address bugs,
• Urgent fixes which need product approval, in order to get merged,
• Docs changes.

Please, if the current pull request addresses a bug fix, label it with the type/bug label.
If it already has the product approval, please add the product-approved label. For docs changes, please add the type/docs label.
If the pull request modifies CI behaviour, please add the type/ci label.
If none of the above applies, please consider removing the backport label and target the next major/minor release.
Thanks!

[jeschkies]

that's my formatted ^^

[grafanabot]

This PR must be merged before a backport PR will be created.

[chaudum]

However, the new last_over_time and first_over_time merge evaluators broke that convention here.

As commented in the code, we should not break the convention.

[cstyan]

discussed on slack with Karsten, LGTM 👍

[grafanabot]

The backport to k210 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-13485-to-k210 origin/k210
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 08615bf7519b31e825903577427f7407194baf74

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-13485-to-k210
# Create the PR body template
PR_BODY=$(gh pr view 13485 --json body --template 'Backport 08615bf7519b31e825903577427f7407194baf74 from #13485{{ "\n\n---\n\n" }}{{ index . "body" }}')
# Create the PR on GitHub
echo "${PR_BODY}" | gh pr create --title 'chore: [k210] fix: Return empty vector instead of nil for empty evaluator.' --body-file - --label 'size/M' --label 'type/bug' --label 'backport' --base k210 --milestone k210 --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-13485-to-k210

# Create a pull request where the `base` branch is `k210` and the `compare`/`head` branch is `backport-13485-to-k210`.

# Remove the local backport branch
git switch main
git branch -D backport-13485-to-k210

[grafanabot]

The backport to k210 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-13485-to-k210 origin/k210
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 08615bf7519b31e825903577427f7407194baf74

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-13485-to-k210
# Create the PR body template
PR_BODY=$(gh pr view 13485 --json body --template 'Backport 08615bf7519b31e825903577427f7407194baf74 from #13485{{ "\n\n---\n\n" }}{{ index . "body" }}')
# Create the PR on GitHub
echo "${PR_BODY}" | gh pr create --title 'chore: [k210] fix: Return empty vector instead of nil for empty evaluator.' --body-file - --label 'size/M' --label 'type/bug' --label 'backport' --base k210 --milestone k210 --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-13485-to-k210

# Create a pull request where the `base` branch is `k210` and the `compare`/`head` branch is `backport-13485-to-k210`.

# Remove the local backport branch
git switch main
git branch -D backport-13485-to-k210