Add x-nullable to paginator response fields

Closes #263.

src/drf_yasg/inspectors/query.py

@@ -71,8 +71,8 @@ class DjangoRestResponsePagination(PaginatorInspector):
                 type=openapi.TYPE_OBJECT,
                 properties=OrderedDict((
                     ('count', openapi.Schema(type=openapi.TYPE_INTEGER) if has_count else None),
-                    ('next', openapi.Schema(type=openapi.TYPE_STRING, format=openapi.FORMAT_URI)),
-                    ('previous', openapi.Schema(type=openapi.TYPE_STRING, format=openapi.FORMAT_URI)),
+                    ('next', openapi.Schema(type=openapi.TYPE_STRING, format=openapi.FORMAT_URI, x_nullable=True)),
+                    ('previous', openapi.Schema(type=openapi.TYPE_STRING, format=openapi.FORMAT_URI, x_nullable=True)),
                     ('results', response_schema),
                 )),
                 required=['results']

tests/reference.yaml

@@ -75,9 +75,11 @@ paths:
               next:
                 type: string
                 format: uri
+                x-nullable: true
               previous:
                 type: string
                 format: uri
+                x-nullable: true
               results:
                 type: array
                 items: