[BUG] NPE when bulk ingest using ingest pipeline without processors

[chishui]

Describe the bug

Got NPE when bulk ingest using ingest pipeline without processors

	"index": {
		"_index": "myindex",
		"_id": null,
		"status": 500,
		"error": {
			"type": "null_pointer_exception",
			"reason": "Cannot invoke \"java.util.List.size()\" because \"results\" is null"
		}
	}

Related component

Indexing

To Reproduce

1. Create an ingest pipeline without any processors

PUT /_ingest/pipeline/test_empty
{
  "description": "This pipeline has no processors",
    "processors": [
    ]
}

2. Use bulk API to ingest docs

POST /_bulk?pipeline=test_empty
{ "index": { "_index": "myindex"} },
{"text":"Generally, an answer is a reply to a question."}

Expected behavior

Documents should be indexed.

Additional Details

Plugins
Please list all plugins currently enabled.

Screenshots
If applicable, add screenshots to help explain your problem.

Host/Environment (please complete the following information):

• OS: [e.g. iOS]
• Version 2.16

Additional context
Add any other context about the problem here.

[chishui]

Fixed in #15033