Fix missing cases of empty_string canonicalization

src/factory.cc

Index: src/factory.cc
diff --git a/src/factory.cc b/src/factory.cc
index a876f9f6fc3ae0ec933f4bdffc848903cab95b8d..24f3a0ada74d549b4e368fd8328bb63366cc41c1 100644
--- a/src/factory.cc
+++ b/src/factory.cc
@@ -820,6 +820,7 @@ MaybeHandle<String> Factory::NewExternalStringFromOneByte(
   if (length > static_cast<size_t>(String::kMaxLength)) {
     THROW_NEW_ERROR(isolate(), NewInvalidStringLengthError(), String);
   }
+  if (length == 0) return empty_string();
 
   Handle<Map> map;
   if (resource->IsCompressible()) {
@@ -844,6 +845,7 @@ MaybeHandle<String> Factory::NewExternalStringFromTwoByte(
   if (length > static_cast<size_t>(String::kMaxLength)) {
     THROW_NEW_ERROR(isolate(), NewInvalidStringLengthError(), String);
   }
+  if (length == 0) return empty_string();
 
   // For small strings we check whether the resource contains only
   // one byte characters.  If yes, we use a different string map.