Fix bug saving falsy attributes

lib/db.js

@@ -654,7 +654,7 @@ function saveDevice(deviceId, deviceData, isNew, sessionTimestamp, callback) {
           for (const attrName of Object.keys(diff[1])) {
             if (
               diff[1][attrName][1] != null &&
-              (!diff[2] || !diff[2][attrName] || !diff[2][attrName][1])
+              (!diff[2] || !diff[2][attrName] || diff[2][attrName][1] == null)
             ) {
               update["$unset"][path.concat(`_${attrName}`).join(".")] = 1;
               if (attrName === "value") {