Fix to check if /tmp/dstat.csv isn't empty to avoid "negative length -xxxx given" error

Previously, it occured that /tmp/dstart.csv was not re-created automatically in a certain situation.

[ NG ]
1. Truncate /tmp/dstat.csv (echo '' > /tmp/dstat.csv), "negative length -xxxx given" error occurs,
   and /tmp/dstart.csv is created again.
2. Remove /tmp/dstat.csv, but /tmp/dstart.csv is not created again.
3. Additionally, when stopping fluentd, "No such file or directory - /tmp/dstat.csv" error occurs.

[ OK (1) ]
1. Remove /tmp/dstat.csv, and /tmp/dstart.csv is created again.

[ OK (2) ]
1. Truncate /tmp/dstat.csv, "negative length -xxxx given" error occurs,
   and /tmp/dstart.csv is created again.

[ OK (3) ]
1. Remove /tmp/dstat.csv, and /tmp/dstart.csv is created again.
2. Truncate /tmp/dstat.csv, "negative length -xxxx given" error occurs,
   and /tmp/dstart.csv is created again.