Move FontFaceSet.{check,load}'s argument default into IDL file

Source/core/css/FontFaceSet.idl

Index: Source/core/css/FontFaceSet.idl
diff --git a/Source/core/css/FontFaceSet.idl b/Source/core/css/FontFaceSet.idl
index 03f9058dfc85e768cbf836b08b80c86d717bf6cd..31ae4fc8d0cc9d2c446b0967cc8dbe1a3f4a72ee 100644
--- a/Source/core/css/FontFaceSet.idl
+++ b/Source/core/css/FontFaceSet.idl
@@ -40,8 +40,8 @@ enum FontFaceSetLoadStatus { "loading", "loaded" };
     attribute EventHandler onloadingdone;
     attribute EventHandler onloadingerror;
 
-    [RaisesException] boolean check(DOMString font, optional DOMString text = null);
-    [CallWith=ScriptState] Promise load(DOMString font, optional DOMString text = null);
+    [RaisesException] boolean check(DOMString font, optional DOMString text = " ");
+    [CallWith=ScriptState] Promise load(DOMString font, optional DOMString text = " ");
     [MeasureAs=FontFaceSetReady, CallWith=ScriptState] Promise ready();
 
     [RaisesException] void add(FontFace fontFace);