fix(reactotron-redux): return empty keys for primitive state paths

[giaBaoJS]

Please verify the following:

• yarn build-and-test:local passes
• I have added tests for any new features, if relevant
• README/docs are not affected by this internal payload correction

Describe your PR

state.keys.request currently forwards undefined when a requested Redux path resolves to a primitive or is invalid. That violates StateKeysResponsePayload.keys: string[] and can break desktop consumers that iterate the field.

This returns an empty key list for non-object paths. The null guard is intentional: typeof null === "object", so checking only typeof would call Object.keys(null) and throw. Object paths still use Object.keys, while state.values.request keeps forwarding the original value unchanged.

Fixes #1599.

Verification

• yarn build-and-test:local
• yarn workspace reactotron-redux test --runInBand — 8 suites, 53 passed, 3 todo
• Packed reactotron-redux and inspected the compiled CommonJS handler
• Ran the real Redux handler into the desktop StateKeysResponseCommand; a primitive path produced keys: [] and rendered the empty-state message without entering the error boundary
• Counterfactual: reverting only the source fix with the final tests kept fails three cases (undefined for primitive and invalid paths, plus Object.keys(null)); restoring the fix passes