504 bug on empty array in field options

```
__PACKAGE__->wing_fields(
some_field => {
  dbic => ..
 ...
  options => sub { return [] },
},
);
```

Passing a sub ref to options, which then ultimately returns an empty array causes a 504 error. It's bad for it to return an empty array anyway. It should never be done. But even so, it should not be a 504 error.