Fix types corresponding to nullable column types

[mjuchli-da]

Currently we have a few occurrences where we use type T | undefined instead of T | null.

In the context of Kysely:
undefined means “property absent from the object, not database NULL.
Kysely converts undefined to “skip this column in INSERT/UPDATE,” not to “write NULL”.

[jaspervdj-da]

I'll add that T | undefined is a bit more ergonomic for libraries (since the user can omit those properties instead of writing null explicitly), so I think the right thing to do is convert undefined <-> null in the functions that convert to and from the XyzTable types, e.g.:

splice-wallet-kernel/core/wallet-store-sql/src/schema.ts

Line 116 in 4f27c3f

export const toNetwork = (table: NetworkTable): Network => {