Feat: kusion should correctly handle empty spec in preview

[ruquanzhao]

What would you like to be added?

Currently, Kusion will throw a nil with logging No resource change found in this stack... for an empty spec during preview and apply.

However, passing an empty spec during the preview is a possible user path, and the result of the preview should indicate that all resources will be deleted.
Related Code:

kusion/pkg/server/manager/stack/execute.go

Lines 201 to 207 in a738404

	// return immediately if no resource found in stack
	// todo: if there is no resource, should still do diff job; for now, if output is json format, there is no hint
	if sp == nil || len(sp.Resources) == 0 {
	logutil.LogToAll(logger, runLogger, "Info", "No resource change found in this stack...")
	return nil, nil
	}

kusion/pkg/cmd/preview/preview.go

Lines 243 to 249 in a738404

	// return immediately if no resource found in stack
	if spec == nil || len(spec.Resources) == 0 {
	if o.Output != jsonOutput {
	fmt.Println(pretty.GreenBold("\nNo resource found in this stack."))
	}
	return nil
	}

Why is this needed?

It is actually valid to pass an empty spec for a preview, and Kusion should handle the preview result correctly.
This change should not affect how apply and destroy handle an empty spec.

[7h3-3mp7y-m4n]

hey @ruquanzhao is it okay if I take this ?

[ruquanzhao]

@7h3-3mp7y-m4n Of course! 🎉🎉🎉

[ruquanzhao]

Hi, @7h3-3mp7y-m4n Have you started this? If there are any problems, we can discuss them directly here.😀

[7h3-3mp7y-m4n]

Hey @ruquanzhao — I've already taken a look at the issue and put together a fix. Just shared the PR with you. I'd really appreciate it if you could take a look when you get a chance!