Usage example in error message incomplete. Just warn about undef instead.

```
$ cat e
use Text::CSV qw( csv );
my $aoa;
csv (in => $aoa, out => "file.csv");
$ perl -w e
usage: my $aoa = csv (in => $file); at e line 3.
```

Here according to the usage message, it looks like "in" can only be used
with files. But in fact it can be used with much more.

The problem is an undef $aoa. So it should in fact warn about that instead.
