Add nil check for groupSnapshotContent in deleteCSIGroupSnapshotOperaion and unit tests

[manishym]

• Add a check to ensure groupSnapshotContent is not nil in deleteCSIGroupSnapshotOperation to prevent panics.
• Enhance error handling for missing SnapshotHandle in VolumeSnapshotContent objects during deletion.
• Create initial unit tests in groupsnapshot_helper_test.go to cover cases where groupSnapshotContent is nil or incomplete.

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change
/kind bug
/kind cleanup
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:
Before fix:

[Running: /usr/local/go/bin/go test -timeout 30s -run ^TestDeleteCSIGroupSnapshotOperation$ [github.com/kubernetes-csi/external-snapshotter/v8/pkg/sidecar-controller](http://github.com/kubernetes-csi/external-snapshotter/v8/pkg/sidecar-controller)]
--- FAIL: TestDeleteCSIGroupSnapshotOperation (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=0x105c44fd4]
goroutine 7 [running]:
testing.tRunner.func1.2({0x1060d4860, 0x107102300})
        /Users/manish/.gvm/pkgsets/go1.22.5/global/pkg/mod/golang.org/[email protected]/src/testing/testing.go:1631 +0x1c4
testing.tRunner.func1()
        /Users/manish/.gvm/pkgsets/go1.22.5/global/pkg/mod/golang.org/[email protected]/src/testing/testing.go:1634 +0x33c
panic({0x1060d4860?, 0x107102300?})
        /Users/manish/.gvm/pkgsets/go1.22.5/global/pkg/mod/golang.org/[email protected]/src/runtime/panic.go:770 +0x124
[github.com/kubernetes-csi/external-snapshotter/v8/pkg/sidecar-controller.(*csiSnapshotSideCarController).deleteCSIGroupSnapshotOperation(0x14000094e48](http://github.com/kubernetes-csi/external-snapshotter/v8/pkg/sidecar-controller.(*csiSnapshotSideCarController).deleteCSIGroupSnapshotOperation(0x14000094e48), 0x140000d65a0)
        /Users/manish/work/openshift/external-snapshotter/pkg/sidecar-controller/groupsnapshot_helper.go:263 +0x404
[github.com/kubernetes-csi/external-snapshotter/v8/pkg/sidecar-controller.TestDeleteCSIGroupSnapshotOperation(0x140002df520)](http://github.com/kubernetes-csi/external-snapshotter/v8/pkg/sidecar-controller.TestDeleteCSIGroupSnapshotOperation(0x140002df520))
        /Users/manish/work/openshift/external-snapshotter/pkg/sidecar-controller/groupsnapshot_helper_test.go:50 +0x1d4
testing.tRunner(0x140002df520, 0x10634dda8)
        /Users/manish/.gvm/pkgsets/go1.22.5/global/pkg/mod/golang.org/[email protected]/src/testing/testing.go:1689 +0xec
created by testing.(*T).Run in goroutine 1
        /Users/manish/.gvm/pkgsets/go1.22.5/global/pkg/mod/golang.org/[email protected]/src/testing/testing.go:1742 +0x318
FAIL    [github.com/kubernetes-csi/external-snapshotter/v8/pkg/sidecar-controller](http://github.com/kubernetes-csi/external-snapshotter/v8/pkg/sidecar-controller)        1.298s
FAIL
[Command exited with 1]

After fix:

[Running: /usr/local/go/bin/go test -timeout 30s -run ^TestDeleteCSIGroupSnapshotOperation$ [github.com/kubernetes-csi/external-snapshotter/v8/pkg/sidecar-controller](http://github.com/kubernetes-csi/external-snapshotter/v8/pkg/sidecar-controller)]
ok      [github.com/kubernetes-csi/external-snapshotter/v8/pkg/sidecar-controller](http://github.com/kubernetes-csi/external-snapshotter/v8/pkg/sidecar-controller) 

Which issue(s) this PR fixes:

Fixes #1123

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: manishym
Once this PR has been reviewed and has the lgtm label, please assign jsafrane for approval. For more information see the Kubernetes Code Review Process.

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[k8s-ci-robot]

Hi @manishym. Thanks for your PR.

I'm waiting for a kubernetes-csi member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[xing-yang]

/ok-to-test

[manishym]

/retest

[xing-yang]

Can you please rebase?