fix(@Persist): prevent null auto-vivification on read operations

[apankov1]

Summary

The @Persist proxy's get() handler was incorrectly auto-vivifying null and undefined values to {} during read operations. This caused:

• Data corruption: Nullable domain fields became empty objects
• D1 serialization issues: JSON.stringify({}) produces "[object Object]" strings instead of NULL values in the database
• Referential transparency violation: Reading a property should never mutate the underlying object

Root Cause

In createDeepProxy, when accessing a property that exists but has a null or undefined value, the proxy was creating a new empty object and assigning it back to the target:

// Before: mutates target[key] from null to {}
const prop = Reflect.get(target, key);
if (typeof prop === 'object' && prop \!== null) { ... }
// Falls through to auto-vivification for null/undefined

Fix

Return null and undefined values as-is without wrapping or auto-vivification:

const prop = Reflect.get(target, key);

// Return null/undefined as-is - these are intentional values
// Do NOT auto-vivify on read operations
if (prop === null || prop === undefined) {
    return prop;
}

Test Plan

• Added regression test: reading null properties returns null, not {}
• Added test: spreading proxied objects preserves null values
• Added test: underlying object is not mutated on read
• Added test: real-world D1 journaling scenario with nullable fields
• Added test: undefined values preserved correctly
• Added test: falsy but valid values (0, false, "") preserved
• Updated persist example to demonstrate nullable field usage

Related

• fix(@Persist): prevent heap overflow in error handler #94 - Separate fix for error handler heap overflow in same code area

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Code in this PR was AI-assisted and exercised against a pet project that experienced the bug.