Fix joinRoom failing when the roomviewstore state changes.

[Half-Shot]

Having investigated this, the bug is that our destructuring in:

const { roomAlias, roomId = payload.roomId, viaServers = [] } = this.state;

does not account for null or other falsey values, which means that this.state.roomId = null takes precedence over payload.roomId.

Checklist

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• Tests written for new code (and old code if feasible).
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