Fix TestDiffEmptyArgClientError O:)

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

api/client/container/diff.go

@@ -34,6 +34,9 @@ func NewDiffCommand(dockerCli *client.DockerCli) *cobra.Command {
 }
 
 func runDiff(dockerCli *client.DockerCli, opts *diffOptions) error {
+	if opts.container == "" {
+		return fmt.Errorf("Container name cannot be empty")
+	}
 	ctx := context.Background()
 
 	changes, err := dockerCli.Client().ContainerDiff(ctx, opts.container)

integration-cli/docker_cli_diff_test.go

@@ -83,5 +83,5 @@ func (s *DockerSuite) TestDiffEnsureDefaultDevs(c *check.C) {
 func (s *DockerSuite) TestDiffEmptyArgClientError(c *check.C) {
 	out, _, err := dockerCmdWithError("diff", "")
 	c.Assert(err, checker.NotNil)
-	c.Assert(strings.TrimSpace(out), checker.Contains, "\"docker diff\" requires exactly 1 argument(s).")
+	c.Assert(strings.TrimSpace(out), checker.Contains, "Container name cannot be empty")
 }