Make FieldType::None() non-nullptr value to avoid undefined behaviour

src/field-type.cc

Index: src/field-type.cc
diff --git a/src/field-type.cc b/src/field-type.cc
index f1cb962b54f322570a41f75856c4baa25a94e9b5..2e4cbfbedde2935d245bd7c47d3e0305211d9eda 100644
--- a/src/field-type.cc
+++ b/src/field-type.cc
@@ -13,7 +13,9 @@ namespace internal {
 
 // static
 FieldType* FieldType::None() {
-  return reinterpret_cast<FieldType*>(Smi::FromInt(0));
+  // Do not Smi::FromInt(0) here or for Any(), as that may translate
+  // as `nullptr` which is not a valid value for `this`.
+  return reinterpret_cast<FieldType*>(Smi::FromInt(2));
 }
 
 // static