Minor driftClient improvement

[andreimontchik]

Would it be possible to add optional chaining to the accountSubscriber.getSpotMarketAccountAndSlot(marketIndex).data calls to avoid possible runtime errors?
https://github.com/drift-labs/protocol-v2/blob/55bcbe47122d85c81fa1dc5df5d50a09e6960ec8/sdk/src/driftClient.ts#L696
https://github.com/drift-labs/protocol-v2/blob/55bcbe47122d85c81fa1dc5df5d50a09e6960ec8/sdk/src/driftClient.ts#L706

this.accountSubscriber.getSpotMarketAccountAndSlot(marketIndex)?.data