[ffigen] `ffi-native` -> `asset` stop logger warning (#449)

pkgs/ffigen/lib/src/config_provider/spec_utils.dart

diff --git a/pkgs/ffigen/lib/src/config_provider/spec_utils.dart b/pkgs/ffigen/lib/src/config_provider/spec_utils.dart
index 149bd9f..56cc872 100644
--- a/pkgs/ffigen/lib/src/config_provider/spec_utils.dart
+++ b/pkgs/ffigen/lib/src/config_provider/spec_utils.dart
@@ -932,8 +932,8 @@
   if (!checkType<YamlMap?>(name, yamlConfig)) {
     return false;
   }
-  // ignore: prefer_void_to_null
-  if (checkType<Null>(name, yamlConfig)) {
+  if (yamlConfig == null) {
+    // Empty means no asset name.
     return true;
   }
   for (final key in (yamlConfig as YamlMap).keys) {