Minor driftClient improvement

[andreimontchik]

Would it be possible to add optional chaining to the accountSubscriber.getSpotMarketAccountAndSlot(marketIndex).data calls to avoid possible runtime errors?

protocol-v2/sdk/src/driftClient.ts

Line 696 in 55bcbe4

public getSpotMarketAccount(

protocol-v2/sdk/src/driftClient.ts

Line 706 in 55bcbe4

public async forceGetSpotMarketAccount(

this.accountSubscriber.getSpotMarketAccountAndSlot(marketIndex)?.data