Fix modifycontroller nil pointer dereference

[AndrewSirenko]

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:

Today, modifying a volume with no initial VolumeAttributesClass will cause the external-resizer to restart before modifying the volume due to a nil pointer dereference in modifycontroller/controller.go.

Reproduction steps: Follow up to step five of aws-ebs-csi-driver modify-volume example. You will see the following logs in the crashed resizer container: csi-resizer-crash-crashed-pod.txt csi-resizer-crash-next-pod.txt

Which issue(s) this PR fixes: Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Fixed external-resizer crash when modifying volume with no previous volume attributes class

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[k8s-ci-robot]

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[AndrewSirenko]

/ok-to-test

[k8s-ci-robot]

@AndrewSirenko: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

In response to [this](https://github.com/kubernetes-csi/external-resizer/pull/375#issuecomment-1976893418): >/ok-to-test Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[xing-yang]

/lgtm /approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: AndrewSirenko, andyzhangx, xing-yang

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