Serialization: Correctly encode structs with no fields

Empty structs were being serialized as `null`, when they should have been serialized as `{}`.

This was due to the type inference on the serializer - where no calls to `field()` would result in the default `null` type.

To solve this, the `serialize(const void* object, const std::initializer_list<Field>&)` inline helper has been promoted to a virtual function (and renamed to `fields()`).
The JSON serializer implementation of this now first sets the object type to `object`, even if there are no fields to serialize.

Added test for this.

Fixes: #10

include/dap/typeof.h

@@ -164,7 +164,7 @@ M member_type(M T::*);
         return d->deserialize(ptr, {__VA_ARGS__});                        \
       }                                                                   \
       bool serialize(Serializer* s, const void* ptr) const override {     \
-        return s->serialize(ptr, {__VA_ARGS__});                          \
+        return s->fields(ptr, {__VA_ARGS__});                             \
       }                                                                   \
     };                                                                    \
     static TI typeinfo;                                                   \