Runtime-variant CRUD 400s when default_model_definition is JSON null (regression from #12970)

## Symptom

After #12970 (BA-6942), all runtime-variant CRUD/search component tests fail with:

```
InvalidRequestError(400, 'Schema validation failed',
'1 validation error for RuntimeVariantModelDefinitionInfo
Input should be a valid dictionary or object to extract fields from
[type=model_attributes_type, input_value=None, input_type=NoneType]')
```

`tests/component/runtime_variant/test_runtime_variant_crud.py` — 6/7 tests fail. Reproduces **deterministically on a clean checkout of current main locally** (`pants test tests/component/runtime_variant/test_runtime_variant_crud.py`), and intermittently on CI (observed twice on an unrelated PR branch, #12979, while other same-base runs passed — the CI-side variance itself is unexplained and worth a look).

## Root cause

`runtime_variants.default_model_definition` is `nullable=False`, but that only forbids **SQL NULL** — a `PydanticColumn` write of Python `None` stores **JSON `null`**, which satisfies the constraint. When a runtime variant is created without a model definition, the row round-trips as `None`, and the adapter introduced in #12970 crashes on it:

```python
# api/adapters/runtime_variant/adapter.py (_data_to_node)
default_model_definition=RuntimeVariantModelDefinitionInfo.model_validate(
data.default_model_definition, # None -> pydantic model_attributes_type error
from_attributes=True,
)
```

`RuntimeVariantData.default_model_definition` is typed non-optional (`ModelDefinitionDraft`), so the None slips through untyped from the row.

## Suggested fix directions

Either (a) make the creation path always materialize a default draft (and backfill existing JSON-null rows via migration), or (b) treat the field as optional end-to-end (`ModelDefinitionDraft | None` on the row/data/DTO and a None-tolerant adapter). (a) matches the intent of "expose model defaults" better.

cc @seedspirit (BA-6942)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Run `pants test tests/component/runtime_variant/test_runtime_variant_crud.py` to reproduce the failure. Read `api/adapters/runtime_variant/adapter.py`, especially `_data_to_node`, then trace the runtime-variant creation path and the row/data/DTO definitions for `default_model_definition`. Done means the CRUD/search component tests pass without JSON-null validation errors, including coverage for variants created without a model definition.

Written by the indexing model from the issue text.