Make semantic non-null the default?

[martinbonnin]

This is a follow up from #63.

GraphQL best practices are currently to use nullable types by default.

With all the work going on in the nullability working group and the advent of semantic non null types, feels like now is a good time to revisit. Should semantically non null types be the default?

This issue is for gathering community feedback and use cases. Would you rather keep the current nullable by default best practice? Or change it to semantically non null by default? Let us know below or come discuss at the next nullability wg meeting!