FilterTransformer对value的判断不合理

com.alibaba.datax.core.transport.transformer.FilterTransformer#evaluate``

columnIndex = (Integer) paras[0];
code = (String) paras[1];
value = (String) paras[2];

if (StringUtils.isEmpty(value)) {
throw new RuntimeException("dx_filter para 2 can't be null");
}

这个判断value值的地方，明明报错是不能为空。但是我先过滤掉空串也不行，是不是考虑改成value==null更更合适

Start at com.alibaba.datax.core.transport.transformer.FilterTransformer#evaluate and inspect how the value parameter is validated. Reproduce the reported behavior for an empty string and a null value, then confirm the intended error condition from the existing message. Done means the validation matches the documented expectation and the relevant behavior is covered by an appropriate test.

Written by the indexing model from the issue text.