Allow prisma-client to take null as input values

[Weakky]

Is your feature request related to a problem? Please describe.

This is related to graphqlgen, and specifically to prisma-labs/graphqlgen#278 (Everything is already clearly explained here)

GraphQL accepts nullable input fields either as undefined or null. As a result, graphqlgen needs to type all input fields as field: type | undefined | null, or field?: type | null

However, prisma-client does not accept null as input values of queries/mutations, probably because it doesn't make sense in the context of the client.

Unfortunately, this prevents users from passing typed arguments with graphqlgen to prisma-client, because undefined | null !== undefined.

[nikolasburk]

Closing as a duplicate of prisma/prisma#3621