GH-2142: Write fields of empty message types as proto bytes

[puskarpeter]

Rationale for this change

Protobuf allows empty message definitions, but Parquet forbids empty groups. Converting a message
that merely contains a field of an empty message type produces a schema with an empty group,
which writer construction rejects with InvalidSchemaException: Cannot write a schema with an empty group. Such fields appear in real-world schemas (deprecated stubs, marker/placeholder
messages), and a single one makes the whole message type unwritable.

What changes are included in this PR?

ProtoSchemaConverter.addMessageField terminates a field whose message type has no fields as a
BINARY column holding the serialized message — the same mechanism PARQUET-1711 uses for
recursion beyond maxRecursion. Since an empty message serializes to zero bytes, the column is
cheap, and field presence still round-trips (null = unset vs empty bytes = set):

• singular field → optional binary stub (or required, per the field);
• repeated field, parquet-specs mode → LIST-wrapped binary via the existing
  addRepeatedPrimitive, so element cardinality survives;
• repeated field, old style → repeated binary;
• map value type → optional binary value inside the key_value group (keys stay typed).

ProtoWriteSupport.createMessageWriter's existing truncated-field check (primitive BINARY where a
message field was declared → BinaryWriter) is generalized to look through the LIST/MAP wrapper
(getGroupType → getContentType), so the writer tree lines up with these schemas.

A message that is empty at the root is still rejected — there is no parent field to hold the
bytes, and a Parquet file with zero columns is not representable.

Are these changes tested?

Yes. New ProtoEmptyMessageTest (new test messages Stub/StubBox in Trees.proto) writes
through the real write path (ProtoParquetWriter → MessageColumnIO, both specs-compliant and
old style) and reads back with GroupReadSupport:

• singular / repeated / map-value empty-message fields round-trip with correct cardinality and
  zero-byte values;
• presence round-trips (set empty message vs unset field);
• an empty root message still fails with InvalidSchemaException ("Cannot write a schema with an
  empty group").

ProtoSchemaConverterTest.testEmptyMessageFields pins the converted schema. The full
parquet-protobuf suite passes (114 tests).

Are there any user-facing changes?

Message types that previously could not be written to Parquet at all now can; fields of empty
message types appear as (possibly LIST/MAP-wrapped) binary columns. No change for schemas that
were previously writable. Error behavior for an empty root message is unchanged.

Closes #2142