code: Map<String, String> filterSigningParams(Map<String, String> signingParams) {
  
          Map<String, String> validParams = new HashMap<>();
  
          for(Map.Entry<String, String> kv : signingParams.entrySet()){
              if (Strings.isNullOrEmpty(kv.getValue())
                      || AlipayField.SIGN.field().equals(kv.getKey())
                      || AlipayField.SIGN_TYPE.field().equals(kv.getKey())){
                  continue;
              }
              validParams.put(kv.getKey(), kv.getValue());
          }
  
          return validParams;
      }
docs: 对签名参数过滤处理，出去值为"", null, sign, signType
  @param signingParams 签名参数
  @return 过滤后的签名参数