return empty array if there are no objects to list

Previously "null" was returned, which does not match the REST backend
specification.

handlers_test.go

@@ -379,6 +379,30 @@ func TestResticErrorHandler(t *testing.T) {
 	}
 }
 
+func TestEmptyList(t *testing.T) {
+	mux, _, _, _, cleanup := createTestHandler(t, Server{
+		AppendOnly: true,
+		NoAuth:     true,
+		Debug:      true,
+	})
+	defer cleanup()
+
+	// create the repo
+	checkRequest(t, mux.ServeHTTP,
+		newRequest(t, "POST", "/?create=true", nil),
+		[]wantFunc{wantCode(http.StatusOK)})
+
+	for i := 1; i <= 2; i++ {
+		req := newRequest(t, "GET", "/data/", nil)
+		if i == 2 {
+			req.Header.Set("Accept", "application/vnd.x.restic.rest.v2")
+		}
+
+		checkRequest(t, mux.ServeHTTP, req,
+			[]wantFunc{wantCode(http.StatusOK), wantBody("[]")})
+	}
+}
+
 func TestListWithUnexpectedFiles(t *testing.T) {
 	mux, _, _, tempdir, cleanup := createTestHandler(t, Server{
 		AppendOnly: true,