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

[HyukjinKwon]

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_HEADERbut 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.

[SparkQA]

Test build #50563 has finished for PR 11023 at commit 5214e8a.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[HyukjinKwon]

retest this please

[SparkQA]

Test build #50564 has finished for PR 11023 at commit 5214e8a.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[falaki]

Thanks @HyukjinKwon this looks good to me.

[HyukjinKwon]

cc @rxin

[rxin]

Thanks. Merging in master.