Use IDL argument default values in modules/encoding/

Source/modules/encoding/TextEncoder.idl

Index: Source/modules/encoding/TextEncoder.idl
diff --git a/Source/modules/encoding/TextEncoder.idl b/Source/modules/encoding/TextEncoder.idl
index dc86fef21bf7e7c28b73df77962e9c24168adc42..d5e9c2f98bdd6f0e4e7bd38b7935317b604f22f9 100644
--- a/Source/modules/encoding/TextEncoder.idl
+++ b/Source/modules/encoding/TextEncoder.idl
@@ -31,11 +31,11 @@
 [
     RuntimeEnabled=EncodingAPI,
     Exposed=Window&Worker,
-    Constructor(optional DOMString utfLabel = null),
+    Constructor(optional DOMString utfLabel = "utf-8"),
     RaisesException=Constructor,
     GarbageCollected,
     MeasureAs=TextEncoderConstructor
 ] interface TextEncoder {
     readonly attribute DOMString encoding;
-    [MeasureAs=TextEncoderEncode] Uint8Array encode(optional ScalarValueString input = null, optional Dictionary options);
+    [MeasureAs=TextEncoderEncode] Uint8Array encode(optional ScalarValueString input = "", optional Dictionary options);
 };