entgql: make connection node non-null

[yukukotani]

ent/ent#3998

What's changed

Generates node field in connection edge types as non-null types instead of nullable types.

Background

According to the spec, node field can be non-null.

An “Edge Type” must contain a field called node. This field must return either a Scalar, Enum, Object, Interface, Union, or a Non-Null wrapper around one of those types. Notably, this field cannot return a list.
https://relay.dev/graphql/connections.htm#sel-FAHFFDCAACEP0xF

On the other hand, edges field in connection types are defined as just a list type that wraps an edge type, which means [Edge] but not [Edge]!.

A “Connection Type” must contain a field called edges. This field must return a list type that wraps an edge type, where the requirements of an edge type are defined in the “Edge Types” section below.
https://relay.dev/graphql/connections.htm#sel-FAFFFDCAACCzEsY

So this PR only changes node field and keeps edges field as it is.

Concerns

My concern is whether we should consider this a breaking change.

GraphQL API clients can accept the transition from nullable to non-null fields without any change or breaking. How about servers? I'm not sure this change would break server implementations using ent.

If so, we can introduce this change as an option.