ui(stats): empty-state with Retry for Redis/Mongo blank payload

[ZhuchkaTriplesix]

Parent

Part of #445

Severity

Low

Problem

After loading clears with no error and null payload → blank colored box, no Retry.

Evidence

• lib/features/redis/redis_view.dart (~229)
• lib/features/mongo/mongo_stats_view.dart (~252) — verify path

Acceptance

• Null/empty successful payload shows empty/error state with Retry
• Distinct from loading and hard-error modes

Suggested fix

Treat as soft error empty state matching other stats views.

[ZhuchkaTriplesix]

Shipped on dev via #462 (Closes did not auto-close because PRs targeted dev).