[FEATURE]: sql.append() should accept undefined

### Describe what you want

It's currently not possible to do something like this:

```
let whereStatement = sql.empty()
whereStatement.append(or(eq(profile.id, "1"),eq(profile.slug, "jon"))
```

That is because `append()` only accepts `SQL` whereas `or()` and `and()` return `SQL | undefined`.

Expected behavior: Pass `undefined` to `append()` and that value will be ignored.