Modern Ecto represents embeds_many/embeds_one as parameterized types
{:parameterized, {Ecto.Embedded, ...}}, which fell through to the catch-all
in type_to_schema/1 and derived to %{}. This hid the nested object's
structure from the model, so it returned null for those fields.

Add clauses for the parameterized Ecto.Embedded form (arrays for :many,
objects for single embeds). Regression test added.