Fix isVerified returning false

which would cause key backups uploads to be missing is_verified
because it was set to `undefined` which would cause the backup to
fail

Fixes https://github.com/vector-im/riot-web/issues/12901

src/crypto/CrossSigning.js

@@ -574,9 +574,9 @@ export class DeviceTrustLevel {
      * @returns {bool} true if this device is verified via any means
      */
     isVerified() {
-        return this.isLocallyVerified() || (
+        return Boolean(this.isLocallyVerified() || (
             this._trustCrossSignedDevices && this.isCrossSigningVerified()
-        );
+        ));
     }
 
     /**