Fix protobuf schema of java function error (#5569)

Signed-off-by: Nxiaolong.ran <ranxiaolong716@gmail.com>
(cherry picked from commit 5c771d20)

pulsar-functions/instance/src/main/java/org/apache/pulsar/functions/source/TopicSchema.java

@@ -151,7 +151,7 @@ public class TopicSchema {
             return (Schema<T>)Schema.KV_BYTES();
 
         case PROTOBUF:
-            return ProtobufSchema.ofGenericClass(clazz, Collections.emptyMap());
+            return ProtobufSchema.ofGenericClass(clazz, new HashMap<>());
 
         default:
             throw new RuntimeException("Unsupported schema type" + type);