api/types/image: make InspectResponse.GraphDriver optional

This change makes the `GraphDriver` field in `image.InspectResponse` optional. This field will only be returned when using moby engine graph drivers as a backend storage implementation. It will be omitted when using the containerd image backend.

Signed-off-by: Austin Vazquez <austin.vazquez@docker.com>

api/swagger.yaml

@@ -2057,6 +2057,7 @@ definitions:
         format: "int64"
         example: 1239828
       GraphDriver:
+        x-nullable: true
         $ref: "#/definitions/DriverData"
       RootFS:
         description: |

api/types/image/image_inspect.go

@@ -111,7 +111,7 @@ type InspectResponse struct {
 
 	// GraphDriver holds information about the storage driver used to store the
 	// container's and image's filesystem.
-	GraphDriver storage.DriverData
+	GraphDriver *storage.DriverData `json:"GraphDriver,omitempty"`
 
 	// RootFS contains information about the image's RootFS, including the
 	// layer IDs.