[BUG]

[fxieatlassian]

What is the bug?
A clear and concise description of the bug.
It looks like this is a bug in the LTR plugin, leading to bad request of params is empty because it's required but doesn't serialise if it is empty. opensearch-learning-to-rank-base/src/main/java/com/o19s/es/ltr/query/StoredLtrQueryBuilder.java at main · opensearch-project/opensearch-learning-to-rank-base makes the params mandatory but opensearch-learning-to-rank-base/src/main/java/com/o19s/es/ltr/query/StoredLtrQueryBuilder.java at main · opensearch-project/opensearch-learning-to-rank-base doesn’t serialize params if they are empty or null.

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

1. send a search request with filter and sltr block, but empty map for the params
2. opensearch will return a bad request

What is the expected behavior?
the expected behaviour is a normal search request, it should return a 200 OK.

What is your host/environment?

• OS: Java
• Version: 2
• Plugins: LTR

Do you have any screenshots?

Do you have any additional context?
Add any other context about the problem.