Fetching preview data in search ML processors may not work if there are query placeholders

[ohltyler]

Currently placeholder values are not persisted. So, when fetching partial data in search request or search response processors when configuring advanced transforms (template, expression), the query with the placeholders is used, and no placeholder values are injected. This can potentially lead to failures depending on what was set as placeholders. If some irrelevant value for some field, it may still work as expected.

Brainstorming few ideas how to resolve this:

1. Offer params to be set / block running if not populated, when running preview in the advanced transform modals
2. Persist param values and require them to be populated in order to save the query
3. Persist param values, but don't require them to be populated. Show warnings if missing parameters detecting when running preview in the advanced transform modals
4. Just add general warning when running preview that if the query has placeholders, could lead to unexpected results.