api/inspect: Fix nil RepoTags and RepoDigests

Make RepoTags and RepoDigests empty arrays instead of nil.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
(cherry picked from commit 1be26e9f0cb954773616585abb25a1227866b79c)
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

api/server/router/image/image_routes.go

@@ -282,6 +282,14 @@ func (ir *imageRouter) toImageInspect(img *image.Image) (*types.ImageInspect, er
 		comment = img.History[len(img.History)-1].Comment
 	}
 
+	// Make sure we output empty arrays instead of nil.
+	if repoTags == nil {
+		repoTags = []string{}
+	}
+	if repoDigests == nil {
+		repoDigests = []string{}
+	}
+
 	return &types.ImageInspect{
 		ID:              img.ID().String(),
 		RepoTags:        repoTags,