LibJS: Don't use null DFS in {Import,Export}Entry

Author: https://github.com/DanShaders
Commit: fb2c929310
Pull-request: https://github.com/SerenityOS/serenity/pull/22926
Reviewed-by: https://github.com/ADKaster ✅
Reviewed-by: https://github.com/davidot ✅

Userland/Libraries/LibJS/AST.cpp

@@ -1530,8 +1530,8 @@ void ExportStatement::dump(int indent) const
     print_indent(indent + 1);
     outln("(ExportEntries)");
 
-    auto string_or_null = [](ByteString const& string) -> ByteString {
-        if (string.is_empty()) {
+    auto string_or_null = [](Optional<DeprecatedFlyString> const& string) -> ByteString {
+        if (!string.has_value()) {
             return "null";
         }
         return ByteString::formatted("\"{}\"", string);