src: use std::nullopt instead of {}

It expresses the intent in a clearer way.

Refs: nodejs#48191 (comment)
Signed-off-by: Darshan Sen <raisinten@gmail.com>

src/node_sea.cc

@@ -292,12 +292,12 @@ std::optional<std::string> GenerateCodeCache(std::string_view main_path,
 
   Local<String> filename;
   if (!String::NewFromUtf8(isolate, main_path.data()).ToLocal(&filename)) {
-    return {};
+    return std::nullopt;
   }
 
   Local<String> content;
   if (!String::NewFromUtf8(isolate, main_script.data()).ToLocal(&content)) {
-    return {};
+    return std::nullopt;
   }
 
   std::vector<Local<String>> parameters = {
@@ -312,7 +312,7 @@ std::optional<std::string> GenerateCodeCache(std::string_view main_path,
   if (!contextify::CompileFunction(
            isolate, context, filename, content, std::move(parameters))
            .ToLocal(&fn)) {
-    return {};
+    return std::nullopt;
   }
 
   std::unique_ptr<ScriptCompiler::CachedData> cache{