Allow setting default secret storage key id to null

[florianduros]

Checklist

• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• Sign-off given on the changes (see CONTRIBUTING.md).

Task element-hq/element-web#28977
The goal is to be able to remove the current recovery key.

The spec expects a string for the key however the rust sdk replaces the content of m.secret_storage.default_key with an empty object({}). We went with the same strategy to keep it consistent.

Also, the MatrixClient already authorises to set the account data with an empty object.

matrix-js-sdk/src/client.ts

Lines 4233 to 4244 in 07f97d7

	/**
	* Set account data event for the current user.
	* It will retry the request up to 5 times.
	* @param eventType - The event type
	* @param content - the contents object for the event
	* @returns Promise which resolves: an empty object
	* @returns Rejects: with an error response.
	*/
	public setAccountData<K extends keyof AccountDataEvents>(
	eventType: K,
	content: AccountDataEvents[K] | Record<string, never>,
	): Promise<{}> {

[richvdh]

I'm a bit surprised the explicit type is needed here.

[florianduros]

The inferred type doesn't work really well here:

const newValue: { key?: undefined } | { key: string }

[t3chguy]

It'd likely work fine as const or inline in the setAccountData call

[richvdh]

lgtm otherwise