rawissueasset doesn't allow asset_amount of zero while issueasset does

elements 0.18.1.9

rawissueasset doesn't allow asset_amount of zero:
```
Invalid parameter, asset_amount must be positive
```

Whereas this is ok using `issueasset`. If you use `issueasset 0 1` you get zero asset amount and 1 reissuance token and then you can effectively reissue the asset into existence.

Would be good if you could do the same with `rawissueasset` for cases where people want to issue an asset and want to know the asset id but do not know the amount they need yet.