Fix empty YAML validation for explicit zero values

[Rohithmatham12]

What this changes

• Records whether root-level YAML config keys are empty before strict unmarshalling.
• Keeps the existing empty-root-node guard for cases like querier: and querier: {}.
• Allows explicit zero values inside a non-empty root config, such as flusher: { exit_after_flush: false }.
• Adds a startup config parsing regression test for the reported flusher case.

Fixes #5488.

Testing

• GOCACHE=/private/tmp/cortex-go-cache GOMODCACHE=/private/tmp/cortex-go-mod-cache go test ./cmd/cortex