Util to test if all ptr fields are nil

This is a lead-up to the volumes-as-plugin PR, which has gotten rather large.
I'm having a hard time understanding the general use case for this. I'd expect any time I want such a thing I also want to check for nil slices and nil maps?

If your use-case is checking some property about a struct with multiple fields for plugins, maybe consider naming this function differently or expanding this comment? (and I guess I'd expect the property to be "exactly one is non-nil" but maybe you're using this for something I'm not imagining.)