don't export 0s for undefined values!

src/components/DataExportSection.vue

@@ -131,7 +131,20 @@ export default class DataExportSection extends Vue {
    */
   public async exportDatabase() {
     try {
-      const blob = await db.export({ prettyJson: true });
+      const blob = await db.export({
+        prettyJson: true,
+        transform: (table, value, key) => {
+          if (table === "contacts") {
+            // Dexie inserts a number 0 when some are undefined, so we need to totally remove them.
+            Object.keys(value).forEach(prop => {
+              if (value[prop] === undefined) {
+                delete value[prop];
+              }
+            });
+          }
+          return { value, key };
+        },
+      });
       const fileName = `${db.name}-backup.json`;
 
       if (this.platformCapabilities.hasFileDownload) {