Support for null literal

[olirice]

It is not currently possible to update a value to null or insert a record with a null value where a default exists.

The GraphQL spec supports a null literal graphql/graphql-spec#83
so null does not need to be treated as absent in all cases

Note:
it is not clear to me what the behavior it expected to be in all cases from the spec
need to look at other implementations to make sure they all agree

For example, I assume passing null to the after argument during pagination should be ignored? If not, what is the distinction

[olirice]

resolved by #290