Return default values for unset optional fields instead of throwing

[merlimat]

Summary

• Optional fields now return type-appropriate default values when not set, matching standard Protobuf behavior
• Only required fields throw IllegalStateException when accessed unset
• Fixed clear() to properly reset all field types (numbers, booleans, enums, strings, bytes, messages) back to defaults

Motivation

Code that works with standard Protobuf relies on optional field getters returning default values (0, false, "", empty bytes, first enum value, default message instance). LightProto was throwing IllegalStateException instead, breaking compatibility.

Changes

Code generator (6 files):

• LightProtoNumberField / LightProtoBooleanField / LightProtoEnumField: Changed throw condition from !hasImplicitPresence && !isDefaultValueSet to isRequired(); fixed clear() to always reset
• LightProtoStringField: Required fields throw; implicit presence uses bufferLen check; optional without default returns ""
• LightProtoBytesField: Required fields throw; all others return empty bytes/buffer
• LightProtoMessageField: Required fields throw; optional fields lazily create an empty instance

Tests (5 files, 10 new tests):

• Unset optional field access: numbers, strings, bytes, messages (including nested sub-messages)
• clear() resets: all number types, strings, bytes, sub-messages, proto3 fields
• Wire-format verification against protobuf-java after clear

Test plan

• All 199 tests pass (189 existing + 10 new)
• Verified unset optional field access returns defaults for all types
• Verified clear() resets values (not just presence bits) for all types
• Verified sub-message getter returns empty instance without setting presence
• Verified serialization compatibility with protobuf-java after clear