fix: store a list of models that admits a None

[shcheklein]

Stacked on #1976.

is_chain_type accepts list[Child | None], and list[Child] writes, but
building the schema raised:

class Child(dc.DataModel):
    x: int

class Holder(dc.DataModel):
    vals: list[Child | None]

chain.map(h=..., output=Holder)
# TypeError: Cannot recognize type <class 'Child'>

The check that maps a model element to JSON looked at the annotation as written,
and the union arm was all that hid the model. It asks what the arm holds now.

With #1975's flatten fix already in place, all three orders round-trip:

[Child(x=1), None]  ->  [Child(x=1), None]
[None, Child(x=2)]  ->  [None, Child(x=2)]
[None, None]        ->  [None, None]

What stays refused, deliberately

Only a plain Model | None is admitted. Two shapes are stored wrong rather than
not at all, so they keep raising:

annotation	why
list[Annotated[Child, "meta"] | None]	rebuilding a nested model does not look through Annotated, so it comes back as [None, {'x': 1}]
list[RootModel[int] | None]	a root model dumps to its bare value and the reader rebuilds one only from a mapping, so it comes back as [None, 1]

list[RootModel[int]] already reads back as bare ints on main — that is
pre-existing and left alone; what this must not do is admit a spelling main
refuses and then store it wrong.

Every slot of a fixed tuple is checked too, since they share the one column:
tuple[A | None, Annotated[B, "meta"] | None] is refused on the strength of its
second slot, while tuple[A | None, B | None], whose slots both read back as
models, is allowed. A whole-model read hides this class of bug, because Pydantic
revalidates the dict — only a leaf read shows it.

Compatibility

Nothing that already wrote is affected: every annotation here raised before, so
there is no stored form to stay compatible with. Storage is unchanged across the
fourteen annotations checked in #1975, and the declared type changes only for
list[Model | None], which had none.

Stack

1. fix: keep None in an array wherever it sits #1975 — None anywhere in an array
2. fix: recognize every spelling of a nullable array element #1976 — the spellings of a nullable element type
3. this PR — a list of models that admits a None

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