Fix parsing of vector expression

[chaudum]

What this PR does / why we need it:

This PR fixes two bugs where a query with the vector() expression failed because it could not be parsed and returned the error:

error : queries require at least one regexp or equality matcher that does not have an empty-compatible value. For instance, app=~".*" does not meet this requirement, but app=~".+" will

• The case where vector() was used as a LogSelectorExpr, e.g. in label_format(vector(0), "foo", "bar", "", "")
• The case where vector() was used in a VectorAggregationExpr, e.g. in sum(vector(0))

Which issue(s) this PR fixes:

Fixes #8266

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• CHANGELOG.md updated
• Changes that require user attention or interaction to upgrade are documented in docs/sources/upgrading/_index.md

[grafanabot]

The backport to release-2.7.x 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-8448-to-release-2.7.x origin/release-2.7.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 96d522753264de2635cec135a8854b4a945c9945
# Push it to GitHub
git push --set-upstream origin backport-8448-to-release-2.7.x
git switch main
# Remove the local backport branch
git branch -D backport-8448-to-release-2.7.x

Then, create a pull request where the base branch is release-2.7.x and the compare/head branch is backport-8448-to-release-2.7.x.