Minor Description
If the index-jexl-filter plugin is enabled but no configuration is provided in the index.jexl.filter property the plugin throws a RuntimeException. In the same exception message, we advise to either set true or false to index all/none.
This is a case where we can just select a sane default and log a warning, but not stop the entire process. I think this is more consistent with how we approach configuration in general: Only fail if there is an actual error in the configuration (i.e parse error on the expression).