title: Read a csv file using FlatFileItemReader, throwing an exception when encountering an empty column
body: <p>When using the FlatFileItemReader to read a csv file, a column mapping type is Int, but this column is null in the csv file (eg:<code>6321517,Jack, 1,,</code> . The last two columns are empty).
  An exception(<code>java.lang.NumberFormatException: Unparseable number</code>) is thrown when parsing a file </p>
  
  <p>csv</p>
  
  <pre><code>CUSTR_NBR,SUR_NAME,CHECK_FLAG,RESN_CODE
  6321517,Jack,1,,
  </code></pre>
  
  <p>The first line of data (CUSTR_NBR, SUR_NAME, CHECK_FLAG, RESN_CODE) is parsed at first, so I set <code>.SetLinesToSkip(1)</code>. However, the corresponding "CHECK_FLAG" and "RESN_CODE" cannot be performed normally if the null value is parsed. I believe there is a corresponding configuration item. I looked at the documentation for springbatch and couldn't find any related configuration items.</p>
  
  <p>csvItemReader</p>
  
  <pre class="lang-java prettyprint-override"><code>    @Bean
      @StepScope
      public FlatFileItemReader&lt;InfoDTO&gt; csvItemReader() {
          FlatFileItemReader&lt;InfoDTO&gt; csvItemReader = new FlatFileItemReader&lt;&gt;();
          csvItemReader.setResource(new ClassPathResource("data/charge-off.csv"));
          csvItemReader.setLinesToSkip(1);
  
          DelimitedLineTokenizer tokenizer=new DelimitedLineTokenizer();
          String[] tokens = new String[]{"CUSTR_NBR","SUR_NAME","CHECK_FLAG","RESN_CODE","EMPNO"};
          tokenizer.setNames(tokens);
          DefaultLineMapper&lt;InfoDTO&gt; lineMapper=new DefaultLineMapper&lt;InfoDTO&gt;();
          lineMapper.setLineTokenizer(tokenizer);
          lineMapper.setFieldSetMapper(new InfoFileMapper());
          lineMapper.afterPropertiesSet();
          csvItemReader.setLineMapper(lineMapper);
  
          return csvItemReader;
      }
  </code></pre>
  
  <p>mapper</p>
  
  <pre><code>    public class InfoFileMapper implements FieldSetMapper&lt;ChargeOffBatchDTO&gt; {
          @Override
          public InfoDTO mapFieldSet(FieldSet fieldSet) throws BindException {
  
              if(fieldSet == null){
                  return null;
              }
  
              return new InfoDTO(
                  fieldSet.readString("CUSTR_NBR"),
                  fieldSet.readString("SUR_NAME"),
                  fieldSet.readString("CHECK_FLAG"),
                  fieldSet.readInt("RESN_CODE"),
                  fieldSet.readInt("EMPNO")
              );
          }
      }
  </code></pre>
  
  <p>I need to map the null column to a value of 0. How to configure？</p>