`write-default` null representation issue

### Apache Iceberg version

None

### Query engine

None

### Please describe the bug 🐞

TL;DR:
It's not always possible to set the DEFAULT value for newly written rows to NULL, without also altering what the DEFAULT value for existing rows should be.

Because of the backwards-compatibility of `write-default`, it will default to `initial-default` if it's not present.
To represent `NULL`, the default has to be omitted.

`null` seems to not be an accepted state for either `initial-default` or `write-default`.
The spec doesn't say they're nullable, and the iceberg-rest-fixture Catalog will return 500 if it reads a `initial-default` or `write-default` that is `null`

That means it's impossible to set the `write-default` to NULL, unless `initial-default` is also NULL (omitted)

Table to illustrate the problem:
```
┌──────────────────┬────────────────┬────────────────────┐
│ initial-default │ write-default │ result (new rows) │
├──────────────────┼────────────────┼────────────────────┤
│ 5 │ - │ 5 │
│ - │ 21 │ 21 │
│ 5 │ 21 │ 21 │
│ - │ - │ NULL │
└──────────────────┴────────────────┴────────────────────┘
```

`write-default` is omitted (`-`), but because `initial-default` is non-null, the result won't be NULL

### Willingness to contribute

- [ ] I can contribute a fix for this bug independently
- [ ] I would be willing to contribute a fix for this bug with guidance from the Iceberg community
- [x] I cannot contribute a fix for this bug at this time

Start by reading the Iceberg specification sections defining initial-default and write-default, then reproduce the behavior with the iceberg-rest-fixture Catalog. Determine how a NULL write-default should be represented independently of initial-default; done when the representation is accepted without the Catalog returning a 500 error.

Written by the indexing model from the issue text.