List field with non-nullable items having a `null` (due to a coercing error), then whole field must be `null`

[Leonardo Losoviz]

Implement result coercion behavior defined in the spec, for whenever a List field cannot have nullable values (eg: [String!]):

If a list’s item type is nullable, then errors occurring during preparation or coercion of an individual item in the list must result in a the value null at that position in the list along with a field error added to the response. If a list’s item type is non-null, a field error occurring at an individual item in the list must result in a field error for the entire list.

[Leonardo Losoviz]

Related to:

• #1457