fix(core): pass null to avoid serialization error

[JianJroh]

Summary

Caused by #301. First call to devtoolskit:internal:messages:list passed undefined as the cursor, which the strict JSON wire format rejects. Pass null instead.

Reproduction

1. pnpm play
2. Open browser console — see [DF0020] ... value at "[0]" is a undefined on load.

Additional context

devtools/devframe/packages/devframe/src/rpc/serialization.ts

Lines 66 to 70 in dc4f925

	if (original === undefined) {
	if (Array.isArray(holder))
	throw nonJsonAt(fnName, 'undefined', holder, key)
	return val
	}

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commit: dc4f925