internal/flex: handle nil items in FlattenStringList

[jar-b]

Description

Prior to this change, the FlattenStringList function would attempt to dereference nil items when provided in the source slice. Now nil items are skipped and omitted from the resulting []interface{}{} return value.

Before:

% go test -count=1 ./internal/flex/... -run=TestFlattenStringList
--- FAIL: TestFlattenStringList (0.00s)
    --- FAIL: TestFlattenStringList/nil_item (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
        panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x0 pc=0x10319c57c]

goroutine 9 [running]:
testing.tRunner.func1.2({0x10369e4c0, 0x103ab3520})
        /Users/jaredbaker/sdk/go1.22.6/src/testing/testing.go:1631 +0x1c4
testing.tRunner.func1()
        /Users/jaredbaker/sdk/go1.22.6/src/testing/testing.go:1634 +0x33c
panic({0x10369e4c0?, 0x103ab3520?})
        /Users/jaredbaker/sdk/go1.22.6/src/runtime/panic.go:770 +0x124
github.com/hashicorp/terraform-provider-aws/internal/flex.FlattenStringList(...)
        /Users/jaredbaker/development/terraform-provider-aws/internal/flex/flex.go:107
github.com/hashicorp/terraform-provider-aws/internal/flex.TestFlattenStringList.func1(0x14000799ba0)
        /Users/jaredbaker/development/terraform-provider-aws/internal/flex/flex_test.go:550 +0x9c
testing.tRunner(0x14000799ba0, 0x14000409ea0)
        /Users/jaredbaker/sdk/go1.22.6/src/testing/testing.go:1689 +0xec
created by testing.(*T).Run in goroutine 6
        /Users/jaredbaker/sdk/go1.22.6/src/testing/testing.go:1742 +0x318
FAIL    github.com/hashicorp/terraform-provider-aws/internal/flex       0.433s

After:

% go test -count=1 ./internal/flex/... -run=TestFlattenStringList
ok      github.com/hashicorp/terraform-provider-aws/internal/flex       0.634s

Relations

Closes #34199

[github-actions]

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[ewbankkit]

LGTM 🚀.

CHANGELOG?

[github-actions]

This functionality has been released in v5.63.1 of the Terraform AWS Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!

[github-actions]

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If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.