Bug fix for a PARAMETER_SECTION only containing LOCAL_CALCS or comments.

If the PARAMETER_SECTION passed to `do_admb()` only contains LOCAL_CALCS or comments, then `proc_var()` strips all lines within it away. This causes an error when

```
maxlen0 <- max(sapply(rest2,length))
```

is called, as `rest2` is an empty list.

This was important for me as I wanted to implement parameter scaling, and (as far as I know) the only way to do this is with LOCAL_CALCS inside the PARAMETER_SECTION.

The fix is a little messy but seems to work. Perhaps there is a neater way of doing this.