Return null for key backup state when we haven't checked yet
Return null for key backup state when we haven't checked yet

This allows distinguishing the "unknown" case from "not present".

Part of https://github.com/vector-im/riot-web/issues/13404

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