fix: empty resources in stack will generate panic

[SparkYuan]

1. Does this PR affect any open issues?(Y/N) and add issue references (e.g. "fix #123", "re #123".):

• N
• Y

2. What is the scope of this PR (e.g. component or file name):

apply and preview cmd

3. Provide a description of the PR(e.g. more details, effects, motivations or doc link):

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Other

4. Are there any breaking changes?(Y/N) and describe the breaking changes(e.g. more details, motivations or doc link):

• N
• Y

5. Are there test cases for these changes?(Y/N) select and add more details, references or doc links:

• Unit test
• Integration test
• Benchmark (add benchmark stats below)
• Manual test (add detailed scripts or steps below)
• Other

6. Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

support empty resource in stack in some circumstances

[coveralls]

Pull Request Test Coverage Report for Build 3545812374

• 2 of 8 (25.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 72.326%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/kusionctl/cmd/apply/options.go	1	4	25.0%
pkg/kusionctl/cmd/preview/options.go	1	4	25.0%

Totals
Change from base Build 3523382780:	0.1%
Covered Lines:	4571
Relevant Lines:	6320

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💛 - Coveralls