Common up some more code in the handling of default values for primitive types.

src/components/script/dom/bindings/codegen/CodegenRust.py

@@ -1256,12 +1256,7 @@ for (uint32_t i = 0; i < length; ++i) {
     if defaultValue is not None:
         if isinstance(defaultValue, IDLNullValue):
             assert type.nullable()
-            template = CGWrapper(CGIndenter(CGGeneric(template)),
-                                 pre="if ${haveValue} {\n",
-                                 post=("\n"
-                                       "} else {\n"
-                                       "  ${declName} = None;\n"
-                                       "}")).define()
+            defaultStr = "None"
         else:
             tag = defaultValue.type.tag()
             if tag in numericTags: