Fix alignment bug caused by empty objects

[mikee47]

This PR fixes a bug demonstrated by the following schema:

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "type": "object",
  "properties": {
    "empty": {
      "type": "object",
      "description": "Must preceded 'guard' for checking struct alignment"
    },
    "guard": {
      "type": "integer",
      "default": 555
    }
  }
}

The struct which provides default values is generated as:

struct __attribute__((packed)) Struct {
    ContainedEmpty::Struct empty{};
    int32_t guard{555};
};

Because C++ allocates one byte to empty structures, guard is misaligned.

The code generator is now updated so that empty structs are omitted entirely:

struct __attribute__((packed)) Struct {
    int32_t guard{555};
};