Translate nzchar()

I think `nzchar()` has an ANSI representation as `x <> ""`.

We could also be more precise and take `keepNA` into account:

 - `nzchar(x, keepNA = TRUE)` ➡️  `x <> ""`
 - `nzchar(x, keepNA = FALSE)` ➡️  `(x IS NULL OR x <> "")`