TypeScript client codegen: generate | null for optional pointer fields

[ravilamichhane]

when encore generates typescript types for web clients, pointer fields (e.g. *string) marked as optional are currently generated as field?: string. since these fields can be nil in go (→ null in json), it will be more type safe if the generated type instead be field?: string | null.

[ctzaruba]

agree with that, field?: string | null is correct format

[fredr]

This will be addressed in #2163