backuptask: fix crash when (old) stats object has no copy field

src/backuptask.js

@@ -319,7 +319,7 @@ async function backupBox(backupSite, appBackupsMap, tag, options, progressCallba
     if (!error) {
         stats.copy = { startTime: copyStartTime, duration: Date.now() - copyStartTime };
         // stats object might be null for stopped/errored apps from old versions
-        stats.aggregatedCopy = Array.from(appBackupsMap.values()).filter(s => !!s).reduce((acc, cur) => ({
+        stats.aggregatedCopy = Array.from(appBackupsMap.values()).filter(s => !!s?.copy).reduce((acc, cur) => ({
             startTime: Math.min(acc.startTime, cur.copy.startTime),
             duration: acc.duration + cur.copy.duration,
         }), stats.copy);