Backport of CSI: avoid a nil pointer when handling plugin events into release/1.4.x #15532
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Backport
This PR is auto-generated from #15518 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.
Fixes #15478
If a plugin crashes quickly enough, we can get into a situation where the deregister function is called before it's ever registered. Safely handle the resulting nil pointer in the dynamic registry by not emitting a plugin event, but also update the plugin event handler to tolerate nil pointers in case we wire it up elsewhere in the future.