Error out in human friendly way if mimetype is missing on response

Start in openapi_core/validation/response/validators.py at the response data lookup, then inspect schema/responses/models.py and schema/content/models.py where the missing mimetype reaches fnmatch. Reproduce the None-mimetype case and make the resulting failure human-friendly, with a test showing the expected error.

Currently one gets:

../../../../.local/share/virtualenvs/woocart-api-zie1bQwL/lib/python3.6/site-packages/openapi_core/validation/response/validators.py:37: in validate
    data, data_errors = self._get_data(response, operation_response)
../../../../.local/share/virtualenvs/woocart-api-zie1bQwL/lib/python3.6/site-packages/openapi_core/validation/response/validators.py:53: in _get_data
    media_type = operation_response[response.mimetype]
../../../../.local/share/virtualenvs/woocart-api-zie1bQwL/lib/python3.6/site-packages/openapi_core/schema/responses/models.py:21: in __getitem__
    return self.content[mimetype]
../../../../.local/share/virtualenvs/woocart-api-zie1bQwL/lib/python3.6/site-packages/openapi_core/schema/content/models.py:18: in __getitem__
    if fnmatch.fnmatch(mimetype, key):  
../../../../.local/share/virtualenvs/woocart-api-zie1bQwL/lib/python3.6/fnmatch.py:34: in fnmatch
    name = os.path.normcase(name)                                  
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
                                          
s = None                             
       
    def normcase(s):                                                                                                                                     
        """Normalize case of pathname.  Has no effect under Posix"""
>       s = os.fspath(s)
E       TypeError: expected str, bytes or os.PathLike object, not NoneType