[Fix] use undefined in "done" CreateIterResultObject calls

Per #33 (comment)

spec.emu

@@ -96,15 +96,15 @@ contributors: Jordan Harband
 			1. If Type(_O_) is not Object, throw a *TypeError* exception.
 			1. If _O_ does not have all of the internal slots of a RegExp String Iterator Object Instance (see <emu-xref href="#PropertiesOfRegExpStringIteratorInstances"></emu-xref>), throw a *TypeError* exception.
 			1. If _O_.[[Done]] is *true*, then
-				1. Return ! CreateIterResultObject(*null*, *true*).
+				1. Return ! CreateIterResultObject(*undefined*, *true*).
 			1. Let _R_ be _O_.[[IteratingRegExp]].
 			1. Let _S_ be _O_.[[IteratedString]].
 			1. Let _global_ be _O_.[[Global]].
 			1. Let _fullUnicode_ be _O_.[[Unicode]].
 			1. Let _match_ be ? RegExpExec(_R_, _S_).
 			1. If _match_ is *null*, then
 				1. Set _O_.[[Done]] to *true*.
-				1. Return ! CreateIterResultObject(*null*, *true*).
+				1. Return ! CreateIterResultObject(*undefined*, *true*).
 			1. Else,
 				1. If _global_ is *true*,
 					1. Let _matchStr_ be ? ToString(? Get(_match_, *"0"*)).

spec.md

@@ -61,15 +61,15 @@ All RegExp String Iterator Objects inherit properties from the [%RegExpStringIte
   1. If [Type][type](O) is not Object, throw a **TypeError** exception.
   1. If O does not have all of the internal slots of a RegExp String Iterator Object Instance (see [here](#PropertiesOfRegExpStringIteratorInstances)), throw a **TypeError** exception.
   1. If *O*.[[Done]] is **true**, then
-    1. Return ! [CreateIterResultObject][create-iter-result-object](**null**, **true**).
+    1. Return ! [CreateIterResultObject][create-iter-result-object](**undefined**, **true**).
   1. Let *R* be *O*.[[IteratingRegExp]].
   1. Let *S* be *O*.[[IteratedString]].
   1. Let *global* be *O*.[[Global]].
   1. Let *fullUnicode* be *O*.[[Unicode]].
   1. Let *match* be ? [RegExpExec][regexp-exec](*R*, *S*).
   1. If *match* is **null**, then
     1. Set *O*.[[Done]] to **true**.
-    1. Return ! [CreateIterResultObject][create-iter-result-object](**null**, **true**).
+    1. Return ! [CreateIterResultObject][create-iter-result-object](**undefined**, **true**).
   1. Else,
     1. If *global* is **true**,
       1. Let *matchStr* be ? [ToString][to-string](? [Get][get](*match*, **"0"**)).