Prevent schemas from using nullable type for top-level field definitions

[achou11]

We currently do not define any top-level schema fields as nullable because Protobuf doesn't have a proper representation of null. However, there's a misalignment between this and how SQLite works, where there's no undefined / void column type in SQLite and so optional fields are stored and read as null. In Mapeo Core, after reading values from SQLite (via Drizzle), we update fields with null values to undefined to match the Mapeo Schema definition of an optional field (link).

However, there's currently no protection against us defining a nullable top-level field in the schema, which will cause an issue at the SQLite-to-MapeoCore level because we will replace an intentional null value with undefined as opposed to leaving it untouched. This does not affect schema fields with a nested JSON-like value definition since we store the JSON as-is in SQLite.

Recommendations:

• add a test to prevent maintainers from defining a top-level nullable field on a schema
• update the schema generation implementation to warn/error if it detects a change that introduces this