Upgrade to Prisma ORM 6 - null/undefined behavior change

[tonypapousek]

In the Prisma Client docs (Fields & types -> Null and undefined), it mentions that there's a shift in behavior between Prisma ORM 5.19.1 or earlier and version 5.20.0.

Since many folks rely exclusively on the upgrade guide when bumping major versions, they can easily miss the breaking change.

Please add a section to the "Upgrade to Prisma ORM 6" about this, otherwise devs will likely miss that behavioral change entirely.