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
The DelimitedIF forwards the returned `null` to the DataSourceTask where it is ignored. Hence, there is no checking for input correctness. If the CsvIF was configured with lenient = false, we need to raise an exception here instead of returning null.