fix(operator): Add validation on empty per-tenant limits config

[btaani]

What this PR does / why we need it:
Prior to this PR, the following Lokistack spec used to be valid:

spec:
  limits:
    tenants:
      invalid: {} 

However, having an empty per-tenant limits config generates an empty map into the loki config, which may cause some problems in some components.

The goal of this PR is to validate the per-tenant limits config to check on empty configurations, which reflects on the status of the Lokistack as a degraded condition.

Which issue(s) this PR fixes:
Fixes LOG-5050

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• CHANGELOG.md updated
  • If the change is worth mentioning in the release notes, add add-to-release-notes label
• 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

[JoaoBraveCoding]

apart from the linting error lgtm

[JoaoBraveCoding]

lgtm

[btaani]

Closing this one since we do not need it anymore. A warning is already generated for empty limits config in Loki: 0f2f42d