Describe the bug

The FlightSQL schema definition for CommandGetDbSchemas and CommandGetTables responses does not fully correspond to the protocol due to missing nullability information

CommandGetDbSchemas

catalog_name is expected to be nullable:

Current definition

/// The schema for GetDbSchemas
static GET_DB_SCHEMAS_SCHEMA: Lazy<SchemaRef> = Lazy::new(|| {
    Arc::new(Schema::new(vec![
        Field::new("catalog_name", DataType::Utf8, false),
        Field::new("db_schema_name", DataType::Utf8, false),
    ]))
});

CommandGetTables

catalog_name and db_schema_name are expected to be nullable:

Current definition

/// The schema for GetTables without `table_schema` column
static GET_TABLES_SCHEMA_WITHOUT_TABLE_SCHEMA: Lazy<SchemaRef> = Lazy::new(|| {
    Arc::new(Schema::new(vec![
        Field::new("catalog_name", DataType::Utf8, false),
        Field::new("db_schema_name", DataType::Utf8, false),
        Field::new("table_name", DataType::Utf8, false),
        Field::new("table_type", DataType::Utf8, false),
    ]))
});

To Reproduce

1. Schemas definition could be manually compared to other FlightSQL servers implementations, for example Cpp or Go.

https://github.com/apache/arrow/blob/0e5249b0e8f060f8ee5136579bfeed8c92d0d1ab/cpp/src/arrow/flight/sql/server.cc#L1282-L1286

3. Run arrow-adbc tests pointing to FlightSQL-based server (see Additional context)

Expected behavior

Fields nullability is adjusted to fully correspond the protocol.

Additional context

Discovered when testing FlightSQL based server spiceai compatibility with arrow-adbc. There are tests failing due to schema mismatch, for example

Exception has occurred: CLR/Apache.Arrow.Adbc.C.CAdbcDriverImporter.ImportedAdbcException An exception of type 'Apache.Arrow.Adbc.C.CAdbcDriverImporter.ImportedAdbcException' occurred in Apache.Arrow.Adbc.dll but was not handled in user code: '[FlightSQL] Invalid schema returned for: expected schema: fields: 2 - catalog_name: type=utf8, nullable - db_schema_name: type=utf8, got schema: fields: 2 - catalog_name: type=utf8 - db_schema_name: type=utf8'