Element-R: Fix rust migration when ssss secret are stored not encryted in cache (old legacy behavior)

[BillCarsonFr]

Fixes element-hq/element-desktop#1603

Some old session might have stored the ssss cached secret in clear form (raw UInt8Array). The existing migration code will fail on such sessions with

TypeError: The first argument must be one of type string, Buffer, ArrayBuffer, Array, or Array-like Object. Received type undefined
    ...
    at async getAndDecryptCachedSecretKey (vector://vector/webapp/bundles/1d2e934471ebcb41f3e7/865.js:4549:44)

Updated the migration's getAndDecryptCachedSecretKey to support both cases similar to

matrix-js-sdk/src/crypto/CrossSigning.ts

Line 659 in 4fc6ba8

if (key && key.ciphertext) {

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).

[andybalaam]

Looks good, thanks!