fix : applied nested path construction from null/empty objects in setIn

[tmdeveloper007]

Summary of What Has Been Done

Fixed setIn() in packages/store/src/immutable.ts to correctly construct nested paths when the source object is null or an empty object. Previously, setIn(null, ['a', 'b'], 'value') returned {} instead of { a: { b: 'value' } }, and setIn(null, ['a', 0, 'b'], 'value') returned { a: { '0': [] } } instead of { a: [{ b: 'value' }] }.

Changes Made

packages/store/src/immutable.ts

• When obj is a plain object cloned via { ...obj } and the next key is numeric with Object.keys(clone).length === 0, replace clone with [] to ensure numeric keys create real array elements instead of string-keyed object properties.
• For the terminal step (rest.length === 0), if clone is an array and the key is a string, convert to a plain object before assignment so the property survives JSON serialization.
• These two changes together ensure nested paths are constructed correctly from any root value.

packages/store/src/immutable.test.ts

• Added test: setIn from null builds nested path correctly
• Added test: setIn from null with numeric index creates array

14 tests pass.

Impact it Made

Makes setIn correctly handle cases where state is initialized from null or where paths reference deeply nested keys that have never been set. This is a common pattern in UI state trees and Redux-style stores.

Closes #3193

**Note: Please assign this PR to the tmdeveloper007 account.

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📥 Commits

Reviewing files that changed from the base of the PR and between 48f63a1 and 7ea2f94.

📒 Files selected for processing (5)

• packages/core/src/history/CommandHistory.test.ts
• packages/core/src/history/CommandHistory.ts
• packages/motion/src/interpolate.ts
• packages/store/src/immutable.test.ts
• packages/store/src/immutable.ts

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