Add optional `nullable` declaration to GraphQL arguments

What does this MR do?

In !60139 (merged) @johnhope and @egrieff noticed that it was a bit awkward not having a way to express "you must provide argument X, but it's OK if it's simply null".

We wound up using the ready? method in a mutation as well as marking the argument as not required. This was better, but it felt a bit naff declaring the argument as "not required, but actually required" due to the lack of required argument nullability in the GraphQL spec.

This MR adds a handy nullable: facet to add to argument declarations, which may only be used when an argument is required (if an argument is not required, it is just noise). It uses ready? under the hood. It also updates the setDueDate mutation to use the new nullable: field.

Related to #329617 (closed)

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