Fix KeyError @ fields to JSON:API schema rendering

Author: alida

starlette_jsonapi/openapi.py

@@ -111,9 +111,11 @@ def fields2jsonschema(self, fields, *, ordered=False, partial=None):
                     is_collection(partial) and field_name not in partial
                 ):
                     if isinstance(field_obj, JSONAPIRelationship):
-                        properties['relationships'].setdefault('required', []).append(observed_field_name)
+                        if 'relationships' in properties:
+                            properties['relationships'].setdefault('required', []).append(observed_field_name)
                     else:
-                        properties['attributes'].setdefault('required', []).append(observed_field_name)
+                        if 'attributes' in properties:
+                            properties['attributes'].setdefault('required', []).append(observed_field_name)
 
         jsonschema['required'] = ['type']
         if 'attributes' in properties and 'required' in properties['attributes']: