Fix possible error occur for v-rebuild-cron-jobs

Since crontab was removed on line 613, if no cronjob being added during the process, it will trigger error on line 626, 627 for No such file or directory

Author: clarkchentw

func/main.sh

@@ -610,7 +610,7 @@ sync_cron_jobs() {
     else
         crontab="/var/spool/cron/$user"
     fi
-    rm -f $crontab
+    >$crontab
     if [ "$CRON_REPORTS" = 'yes' ]; then
         echo "MAILTO=$CONTACT" > $crontab
         echo 'CONTENT_TYPE="text/plain; charset=utf-8"' >> $crontab
@@ -1304,4 +1304,4 @@ no_symlink_chmod() {
 
         chmod "${filemode}" "${i}"
     done
-}
+}