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

tests/objects/t3734.nim

@@ -0,0 +1,17 @@
+discard """
+output: "i0"
+"""
+
+type
+  Application = object
+      config: void
+      i: int
+      f: void
+
+proc printFields(rec: Application) =
+  for k, v in fieldPairs(rec):
+    echo k, v
+
+var app: Application
+
+printFields(app)