[SPARK-13260][SQL] count(*) does not work with CSV data source

https://issues.apache.org/jira/browse/SPARK-13260
This is a quicky fix for `count(*)`.

When the `requiredColumns` is empty, currently it returns `sqlContext.sparkContext.emptyRDD[Row]` which does not have the count.

Just like JSON datasource, this PR lets the CSV datasource count the rows but do not parse each set of tokens.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #11169 from HyukjinKwon/SPARK-13260.