Fix libpod API conformance to swagger

* Return empty array when nothing has been pruned.
* Use correct return type swagger doc-comment.

Signed-off-by: Matej Vasek <mvasek@redhat.com>

pkg/api/server/register_images.go

@@ -1069,7 +1069,7 @@ func (s *APIServer) registerImagesHandlers(r *mux.Router) error {
 	// - application/json
 	// responses:
 	//   200:
-	//     $ref: "#/responses/DocsImageDeleteResponse"
+	//     $ref: "#/responses/DocsLibpodPruneResponse"
 	//   500:
 	//     $ref: '#/responses/InternalError'
 	r.Handle(VersionedPath("/libpod/images/prune"), s.APIHandler(libpod.PruneImages)).Methods(http.MethodPost)

pkg/domain/infra/abi/images.go

@@ -57,7 +57,7 @@ func (ir *ImageEngine) Prune(ctx context.Context, opts entities.ImagePruneOption
 		pruneOptions.Filters = append(pruneOptions.Filters, "containers=false")
 	}
 
-	var pruneReports []*reports.PruneReport
+	pruneReports := make([]*reports.PruneReport, 0)
 
 	// Now prune all images until we converge.
 	numPreviouslyRemovedImages := 1

test/apiv2/10-images.at

@@ -218,6 +218,9 @@ if ! grep -q '400 Bad Request' "${TMPD}/headers.txt"; then
     BUILD_TEST_ERROR="1"
 fi
 
+t POST libpod/images/prune 200
+t POST libpod/images/prune 200 length=0 []
+
 cleanBuildTest
 if [[ "${BUILD_TEST_ERROR}" ]]; then
     exit 1