[SPARK-13137][SQL] NullPoingException in schema inference for CSV when the first line is empty

https://issues.apache.org/jira/browse/SPARK-13137

This PR adds a filter in schema inference so that it does not emit NullPointException.

Also, I removed `MAX_COMMENT_LINES_IN_HEADER `but instead used a monad chaining with `filter()` and `first()`.

Lastly, I simply added a newline rather than adding a new file for this so that this is covered with the original tests.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #11023 from HyukjinKwon/SPARK-13137.