Change serialization to omit none(T) fields of sparse objects (#100)

* Change serialization to omit `none(T)` fields of sparse objects

Co-authored-by: quantimnot <quantimnot@users.noreply.github.com>

yaml/annotations.nim

@@ -27,7 +27,8 @@ template sparse*() {.pragma.}
   ## This annotation can be put on an object type. During deserialization,
   ## the input may omit any field that has an ``Option[T]`` type (for any
   ## concrete ``T``) and that field will be treated as if it had the annotation
-  ## ``{.defaultVal: none(T).}``.
+  ## ``{.defaultVal: none(T).}``. Fields of ``none(T)`` value are omitted
+  ## during serialization.
   ##
   ## Example usage:
   ##
@@ -80,4 +81,4 @@ template implicit*() {.pragma.}
   ##     of ckString:
   ##       strVal: string
   ##     of ckInt:
-  ##       intVal: int
+  ##       intVal: int

yaml/serialization.nim

@@ -967,12 +967,17 @@ macro genRepresentObject(t: typedesc, value, childTagStyle: typed) =
         name = $child
         childAccessor = newDotExpr(value, newIdentNode(name))
       result.add(quote do:
-        when not `childAccessor`.hasCustomPragma(transient):
+        template serializeImpl =
           when bool(`isVO`): c.put(startMapEvent())
           c.put(scalarEvent(`name`, if `childTagStyle` == tsNone:
               yTagQuestionMark else: yTagNimField, yAnchorNone))
           representChild(`childAccessor`, `childTagStyle`, c)
           when bool(`isVO`): c.put(endMapEvent())
+        when not `childAccessor`.hasCustomPragma(transient):
+          when hasSparse(`t`) and `child` is Option:
+            if `childAccessor`.isSome: serializeImpl()
+          else:
+            serializeImpl()
       )
     inc(fieldIndex)