test Content unmarshal on nil

[jba]

We have a couple of UnmarshalJSON methods for Content. Check that they don't panic on nil.

[MrGossett]

I’m interested in helping with this.

[MrGossett]

I took the liberty of putting up a draft PR here: #275

Happy to adjust or close the PR at your direction.

[MrGossett]

This should probably be closed now that #275 is merged.

[findleyr]

Indeed, thanks!