csi: prevent panic on volume delete

[lgfa29]

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
	}

[tgross]

LGTM!