[gn] Enable stricter build flags

src/api.cc

Index: src/api.cc
diff --git a/src/api.cc b/src/api.cc
index 9337708dcf0f7b3985b580c2208edde485fc88e5..e976f7478d8b7a8018cfc4105bd57d5ed3cf5a3f 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -9405,7 +9405,7 @@ void debug::GetLoadedScripts(v8::Isolate* v8_isolate,
     i::DisallowHeapAllocation no_gc;
     i::Script::Iterator iterator(isolate);
     i::Script* script;
-    while ((script = iterator.Next())) {
+    while ((script = iterator.Next()) != nullptr) {
       if (!script->IsUserJavaScript()) continue;
       if (script->HasValidSource()) {
         i::HandleScope handle_scope(isolate);