Hadoop(Input)FormatIO should support Void/null key/values

Many Hadoop InputFormat implementations use Void as key/value type and generates null values which causes NullPointerException in https://github.com/apache/beam/blob/master/sdks/java/io/hadoop/input-format/src/main/java/org/apache/beam/sdk/io/hadoop/inputformat/HadoopInputFormatIO.java#L714

HadoopInputFormatIO should ignore these and not clone them.