void object fields are now ignored by codegen and fields/fieldPairs iterator (#10144)

* Codegen now ignores object fields of type void
* Fix `$` bug for objects/tuples where it does not add a comma
* fields/fieldPairs iterators now ignore void types
* Use `isEmptyType` instead of checking for `tyVoid` directly

compiler/semfields.nim

@@ -19,6 +19,9 @@ type
     c: PContext
 
 proc instFieldLoopBody(c: TFieldInstCtx, n: PNode, forLoop: PNode): PNode =
+  if c.field != nil and isEmptyType(c.field.typ):
+    result = newNode(nkEmpty)
+    return
   case n.kind
   of nkEmpty..pred(nkIdent), succ(nkSym)..nkNilLit: result = n
   of nkIdent, nkSym: