Return an empty translation tree for empty KeyValue stores

[OskarEichler]

Summary

Seed the KeyValue translation reduction with an empty hash.

Without an initial value, an empty store produces nil instead of the backend translation-tree contract's empty hash.

Verification

• focused external empty-store model
• 1,607 base-suite runs / 2,674 assertions
• 1,888 Rails-suite runs / 3,020 assertions
• all 46 shipped Ruby files pass syntax checks

[radar]

But why?

[OskarEichler]

This came from the broad dependency audit rather than a concrete application failure. The narrow rationale was to make an empty store conform to the translation-tree shape, but I do not have a real consumer bug or use case that justifies changing behavior. I am closing it rather than asking the project to carry a speculative change.