enable CSV Reader to ignore invalid lines like an empty line at the end and comments - FLINK-1208

Corresponding to issue FLINK-1208:
enable CSV Reader to ignore invalid lines like an empty line at the end, multiple header lines or comments without throwing an exception
Fabian told me to not return null:  "That's what I meant by letting the DelimitedInputFormat handling invalid lines. I would not give the null value back to the DataSourceTask, but instead let the DelimitedInputFormat catch this and try to call readRecord() until a valid record is return and hand that to the DataSourceTask.
I am actually surprised that giving a null value to the data source does not cause a NPE." (cite by Fabian)