Add nil-input regression coverage for filterIndexes

[wangyusheng1985]

Summary

• Add regression coverage for filterIndexes with nil input.
• Keep production behavior unchanged.

Validation

• gofmt -w pkg/app/ops/firestoreindexensurer/indexes_test.go
• go test ./pkg/app/ops/firestoreindexensurer -run '^TestFilterIndexes$' -count=1
• git diff --check

Does this PR introduce a user-facing change?:

• How are users affected by this change: No user-facing change; this only adds unit-test coverage.
• Is this breaking change: No.
• How to migrate (if breaking change): Not applicable.

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[github-actions]

👋 Hi @wangyusheng1985, welcome to PipeCD and thanks for opening your first pull request!

We’re really happy to have you here

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[Copilot]

🟢 Approval recommended

The change is limited to a small, clear unit-test addition that matches the current filterIndexes behavior and does not affect production code.

Pull request overview

Adds a focused regression test to ensure filterIndexes correctly handles a nil indexes input without panicking and preserves the current behavior (returning nil when given nil).

Changes:

• Add a new TestFilterIndexes table-driven case covering indexes == nil with non-empty excludes.

File summaries

File	Description
pkg/app/ops/firestoreindexensurer/indexes_test.go	Adds regression test coverage for filterIndexes when indexes is nil.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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