fix: richtext toDelta cache err when importing snapshot

[zxch3n]

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[Copilot]

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crates/loro-wasm/tests/richtext.test.ts:372

• [nitpick] The initial expectation of an empty delta might be redundant since the subscription callback later verifies the updated delta. Consider removing or adding a comment clarifying its necessity in ensuring test clarity.

expect(textB.toDelta()).toStrictEqual([]);