fix metadata overwrite

src/mixin.ts

@@ -22,14 +22,18 @@ function applyMixin(
   }
   visited.add(sourceClass);
   Object.getOwnPropertyNames(sourceClass.prototype).forEach((name) => {
-    if (!mixedClass.prototype[name]) {
+    if (mixedClass.prototype[name] != null) {
       mixedClass.prototype[name] = sourceClass.prototype[name];
     }
   });
   const schemaKeys = reflector.getArray('SchemaMetaKey', sourceClass);
   for (const name of schemaKeys) {
     const schemaMeta = reflector.get('SchemaMeta', sourceClass, name);
-    if (!schemaMeta) {
+    if (!schemaMeta == null) {
+      continue;
+    }
+    const existingMetadata = reflector.get('SchemaMeta', mixedClass, name);
+    if (existingMetadata != null) {
       continue;
     }
     Metadata.set(