feat: handle empty stack in preview

[7h3-3mp7y-m4n]

What type of PR is this?

/kind feature

What this PR does / why we need it:

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.

Which issue(s) this PR fixes:

Fixes #1419

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Additional documentation e.g., design docs, usage docs, etc.:

[github-actions]

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[7h3-3mp7y-m4n]

And the CI failed!
I'll fix it ASAP!

[ruquanzhao]

Please also change these comments in code. (line 201~line 202)

[ruquanzhao]

And the CI failed! I'll fix it ASAP!

Please ignore Check Markdown links and e2e test, there are some bugs.

And Constraints / Commit Lint (pull_request) is about your commit message, rewrite to feat: handle empty stack in preview will fix it.

[ruquanzhao]

CLA Assistant Lite bot: Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution. You can sign the CLA by just posting a Pull Request Comment same as the below format.

I have read the CLA Document and I hereby sign the CLA

You can retrigger this bot by commenting recheck in this Pull Request

@7h3-3mp7y-m4n By the way, you need to sign the CLA according to the description here, otherwise the PR will not be accepted. 😃