..parser.ast.expression.arithmeic.ArithmeticSubtractExpression 代码错误

输入参数 long2 判断错误

``` java
@Override
public Number calculate(Long long1, Long long2) {
if (long1 == null || long1 == null)
return null;
...
}
```

[ArithmeticSubtractExpression.java#L68](https://github.com/alibaba/cobar/blob/master/server/src/main/parser/com/alibaba/cobar/parser/ast/expression/arithmeic/ArithmeticSubtractExpression.java#L68)

Open server/src/main/parser/com/alibaba/cobar/parser/ast/expression/arithmeic/ArithmeticSubtractExpression.java at line 68 and inspect the calculate method's null handling. Compare it with the surrounding arithmetic expression implementations and run the relevant parser or expression tests; the issue is done when null inputs are checked correctly without changing subtraction behavior.

Written by the indexing model from the issue text.