fixes #20699; generate an empty struct for void type (#20790)

* fixes #20699; generate an empty struct for void type

* Add docs and test case.

tests/objects/tobject_default_value.nim

@@ -476,6 +476,20 @@ template main {.dirty.} =
 
         foo2()
 
+    block: # issue #20699
+      type
+        Either[A,B] = object
+          case kind:bool
+          of false:
+            b: B
+          of true:
+              a: A
+        O = object of RootRef
+
+      proc oToEither(o:O):Either[O,void] =
+        Either[O,void](kind:true,a: o)
+
+      discard oToEither(O())
 
 static: main()
 main()