fix: prevent Write(nil) from panicking

[masonkatz]

Table.Write(nil) panics because reflect.TypeOf(nil) is nil and the implementation immediately calls Kind() on it. This contradicts Write behavior for other non-struct inputs, which are represented as an error row.

Acceptance criteria

1. Add a regression test that calls Write(nil) and demonstrates the intended non-panicking error-row behavior.
2. Update Write to handle a nil argument before accessing reflection type metadata.
3. Ensure nil uses the same observable error-table/annotation behavior as other non-struct values.
4. Run the relevant test suite.