Fix metrics dashboard YAML validation

What does this MR do?

Fixes validation added in !33202 (merged) which do not cover situation when file with dashboard definition does not contain hash as a root element.

Screenshots

from https://gitlab.com/gitlab-org/monitor/monitor-sandbox/-/blob/master/.gitlab/dashboards/empty_dashboard.yml

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content