Describe the bug
When creating or updating a ingest pipeline by PUT _ingest/pipeline/xx, if the definition of the processor is null, then OpenSearch throws null_pointer_exception with 500 http status code.

To Reproduce
Steps to reproduce the behavior:

1. Create a ingest pipeline

PUT _ingest/pipeline/x
{
  "processors": [
    {
      "set": null
    }
  ]
}

then you can see the error:

{
  "error": {
    "root_cause": [
      {
        "type": "null_pointer_exception",
        "reason": "Cannot invoke \"Object.getClass()\" because \"config\" is null"
      }
    ],
    "type": "null_pointer_exception",
    "reason": "Cannot invoke \"Object.getClass()\" because \"config\" is null"
  },
  "status": 500
}

Expected behavior
Validate the definition of the processor, returns 400 bad request error if the definition is null.

Screenshots

Host/Environment (please complete the following information):

• OS: [macOS]
• Version [2.9]