make scheduler accept multiple batch handlers

meilisearch-lib/src/tasks/batch_handlers/empty_handler.rs

@@ -0,0 +1,20 @@
+use crate::tasks::batch::{Batch, BatchContent};
+use crate::tasks::BatchHandler;
+
+/// A sink handler for empty tasks.
+pub struct EmptyBatchHandler;
+
+#[async_trait::async_trait]
+impl BatchHandler for EmptyBatchHandler {
+    fn accept(&self, batch: &Batch) -> bool {
+        matches!(batch.content, BatchContent::Empty)
+    }
+
+    async fn process_batch(&self, batch: Batch) -> Batch {
+        batch
+    }
+
+    async fn finish(&self, _: &Batch) {
+        ()
+    }
+}