groq-builder Make slice(0) nullable by default

[heggemsnes]

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Feature Request

Since we are not really reading validation on "min" for arrays, doing slice(0) should always infer to be nullable. 

Also with document queries, i.e:

`const pageQuery = q.star.filterByType("page").filter("slug.current == $slug").slice(0)`

groq will return null if the document doesn't exist. I feel like this should be the default. 

We can add .nullable() but I think it is more correct to make it default nullable in this scenario and perhaps add a .nonNullable() if necessary.

[scottrippey]

This is a great point. I like the idea of adding a .notNull() assertion too!