fix: Ensure non-null collection schema type

[timkendrick]

What kind of change does this PR introduce?

Bug fix

What is the current behavior?

Exposed GraphQL schema exposes a nullable type for pagable collection wrapper objects

What is the new behavior?

The schema type is now a non-null type

Additional context

note the ! annotations in the collection return types

Before	After

[olirice]

@imor @alaister i think that's safe b/c it was always impossible for it to return null
some codegen tools may break but overall seems fine. wdyt?

[imor]

@olirice Sounds good to me. @timkendrick could you please fix the failing tests.

[timkendrick]

Thanks @olirice @imor

I've pushed an update that fixes the installcheck test suite, let me know if there's anything else I need to do