Major Description
Currently, `null + interval` will become `cast(cast(null as timestamp) + interval) as null`. This is a unexpected behavior and the result should not be of null type.
This weird behavior applies to `null - interval`, `interval + null`, `interval - null` as well.
To change it, I propose to cast the null as the same data type of the other element in the add/subtract:
```
null + interval => cast(null as interval) + interval
null - interval => cast(null as interval) - interval
interval + null=> interval + cast(null as interval)
interval - null => interval - cast(null as interval)
```
