searchMessageText: avoid setting keys=undefined
searchMessageText: avoid setting keys=undefined

This doesn't make any difference to the JSON, but it upsets `expect`.

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