Alerting: Fix default query's data source when no default datasource specified

[konrad147]

What this PR does / why we need it:
This PR fixes the default query's data source when there is no data source marked as default

[grafanabot]

Drone build failed: https://drone.grafana.net/grafana/grafana-enterprise/34093

[grafanabot]

Drone build failed: https://drone.grafana.net/grafana/grafana-enterprise/34106

[soniaAguilarPeiron]

LGTM! 🎉

[grafanabot]

The backport to v9.0.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-55435-to-v9.0.x origin/v9.0.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 20e6e0d59392e5a03b7ceb54bf7d16134a5f1484
# Push it to GitHub
git push --set-upstream origin backport-55435-to-v9.0.x
git switch main
# Remove the local backport branch
git branch -D backport-55435-to-v9.0.x

Then, create a pull request where the base branch is v9.0.x and the compare/head branch is backport-55435-to-v9.0.x.

[grafanabot]

The backport to v9.1.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-55435-to-v9.1.x origin/v9.1.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 20e6e0d59392e5a03b7ceb54bf7d16134a5f1484
# Push it to GitHub
git push --set-upstream origin backport-55435-to-v9.1.x
git switch main
# Remove the local backport branch
git branch -D backport-55435-to-v9.1.x

Then, create a pull request where the base branch is v9.1.x and the compare/head branch is backport-55435-to-v9.1.x.