Backport of csi: prevent panic on volume delete into release/1.4.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #18234 to be assessed for backporting due to the inclusion of the label backport/1.4.x.

The below text is copied from the body of the original PR.

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When a CSI volume is deleted when its plugin is not running, the function volAndPluginLookup returns a nil plugin value resulting a panic in the request handler.

Note to reviewers: the other place where volAndPluginLookup is used already checks for a nil plugin.

nomad/nomad/csi_endpoint.go

Lines 513 to 518 in e21ab7d

	// Some plugins support controllers for create/snapshot but not attach. So
	// if there's no plugin or the plugin doesn't attach volumes, then we can
	// skip the controller publish workflow and return nil.
	if plug == nil || !plug.HasControllerCapability(structs.CSIControllerSupportsAttachDetach) {
	return nil
	}