Move this logic

It used to make sense for this to live in the ingest service, because
we avoided allocating an ingest document if the pipeline was
empty. Now we already have the document regardless, so this can just
live in IngestDocument anyway.

In any case, this would be a rare and unusual thing to have happen at
all. I don't want to drop the logic completely, but I'm also not
worried about the performance implications of where it lives.

server/src/main/java/org/elasticsearch/ingest/IngestDocument.java

@@ -829,6 +829,12 @@ private static Set<String> getAllFields(Map<String, Object> input, String prefix
      * @param handler handles the result or failure
      */
     public void executePipeline(Pipeline pipeline, BiConsumer<IngestDocument, Exception> handler) {
+        // shortcut if the pipeline is empty
+        if (pipeline.getProcessors().isEmpty()) {
+            handler.accept(this, null);
+            return;
+        }
+
         if (executedPipelines.add(pipeline.getId())) {
             Object previousPipeline = ingestMetadata.put("pipeline", pipeline.getId());
             pipeline.execute(this, (result, e) -> {

server/src/main/java/org/elasticsearch/ingest/IngestService.java

@@ -907,11 +907,6 @@ static String getProcessorName(Processor processor) {
     }
 
     private void innerExecute(final IngestDocument ingestDocument, final Pipeline pipeline, final BiConsumer<Boolean, Exception> handler) {
-        if (pipeline.getProcessors().isEmpty()) {
-            handler.accept(true, null);
-            return;
-        }
-
         ingestDocument.executePipeline(pipeline, (result, e) -> {
             if (e != null) {
                 handler.accept(true, e);