Fix the error about pagerank when $COMPRESS -eq 0

When $COMPRESS -eq 0, COMPRESS_OPT is an unbound variable. And '$HADOOP_EXECUTABLE jar ${DATATOOLS} HiBench.DataGen ${OPTION} ${COMPRESS_OPT}' can not run.
![pagerank](https://cloud.githubusercontent.com/assets/1490540/4041117/6d6b20bc-2cf1-11e4-8739-0341b64afced.png)

I think it is better to use `COMPRESS_OPT=""`