text: Title: 'NoneType' object has no attribute 'description'
  Body: Firstly, thanks for this great library.
  
  Now, in:
  
  https://github.com/axnsan12/drf-yasg/blob/2ef7cfbfe369a55e8b68e574bf20fe32d40cac38/src/drf_yasg/inspectors/query.py#L49
  
  If we can't find out which type it is, we default to string.
  
  However, later, we try to ask the schema description:
  
  https://github.com/axnsan12/drf-yasg/blob/2ef7cfbfe369a55e8b68e574bf20fe32d40cac38/src/drf_yasg/inspectors/query.py#L51
  
  My field that is getting passed in, is from https://github.com/django-money/django-money, has no schema:
  
  ``` python
  ipdb> field
  Field(name=u'currency', required=False, location='query', schema=None, description=u'', type=None, example=None)
  ipdb> type(field.schema)
  <type 'NoneType'>
  ipdb> field.schema.description
  *** AttributeError: 'NoneType' object has no attribute 'description'
  ```
  
  Not sure what to do exactly but it seems that we should make a `field.schema is None` check?