Make getAccountDataFromServer return null if not found
Make getAccountDataFromServer return null if not found

Callers assume that if the account data isn't there, the method will
return null rather than throw an exception. This meant that this only
actually worked in these cases when the sync had completed and it was
using the locally stored account data, so this ceased to be a
problem in practice when we made it wait for the initial sync to finish
before entering the security setup flow.

Fixes https://github.com/vector-im/riot-web/issues/13169

matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk,matrix-org/matrix-js-sdk