Handling $ref to other object in schemas

If in schema there is only a reference to other project, generation in pdf format raise an exception.

For instance in : 

```yaml
    SampleLevelTwoRef2:
      $ref: '#/components/schemas/SampleLevelTwoD'
```

Exception : 

`org.apache.fop.fo.ValidationException: "fo:table-body" is missing child elements. Required content model: marker* (table-row+|table-cell+) (See position 457:6)`