[BUG] Rules search returns 'Custom rules error' when there are no custom rules

[jovancacvetkovic]

What is the bug?
Rules search returns 'Custom rules error' when there are no custom rules. The status code returned is 500.

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

1. Deploy OpenSearch, start, and open the application in a browser.
2. The first page that loads is the overview page
3. There is a backend API call to http://localhost:5601/_plugins/_security_analytics/rules/_search?prePackaged=false
4. The result of the request is
   Original server response: statusCode: 500, response: '{"error":{"root_cause":[{"type":"security_analytics_exception","reason":"Custom rule index doesnt exist. Please create custom rules first."}],"type":"security_analytics_exception","reason":"Custom rule index doesnt exist. Please create custom rules first.","caused_by":{"type":"exception","reason":"java.lang.IllegalArgumentException: Custom rule index doesnt exist. Please create custom rules first."}},"status":500}',

What is the expected behavior?
This should not be an error but a response with status 200 with an empty array response.

Do you have any screenshots?