fix: Context.EmbedObject returns a broken json when object is empty

[rerorero]

This PR fixes an issue with Context.EmbedObject() that produces invalid JSON when

• the given object's MarshalZerologobject adds no fields (empty object)
• and the parent context has existing fields.

When MarshalZerologObject adds no fields (e.g., all fields are conditionally skipped), an extra comma is inserted:

  {"level":"info","parent_field":"parent_value",,"message":"test"}