[BUG] model search api fails when using _source_includes/_source_excludes

[jmazanec15]

What is the bug?
Model search api fails when using query filters _source_includes/_source_excludes

How can one reproduce the bug?
Steps to reproduce the behavior:

1. Create a model
2. Search for that model with the following params:

curl "localhost:9200/_plugins/_knn/models/_search?pretty&_source_excludes=model_blob"

{
  "error" : {
    "root_cause" : [
      {
        "type" : "illegal_argument_exception",
        "reason" : "Cannot construct model in state CREATED when model binary is null. State must be either TRAINING or FAILED"
      }
    ],
    "type" : "illegal_argument_exception",
    "reason" : "Cannot construct model in state CREATED when model binary is null. State must be either TRAINING or FAILED"
  },
  "status" : 400
}

curl "localhost:9200/_plugins/_knn/models/_search?pretty&_source_includes=model_id"

{
  "error" : {
    "root_cause" : [
      {
        "type" : "illegal_argument_exception",
        "reason" : "Invalid engine type: null"
      }
    ],
    "type" : "illegal_argument_exception",
    "reason" : "Invalid engine type: null"
  },
  "status" : 400
}

What is the expected behavior?
Expected query results to return filtered hits