[usability] have a noticeable flag/message when Granted Fields is empty

[elasticmachine]

Original comment by @AlonaNadler:

The customer went through a long debugging session since the * in granted fields was accidentally removed and the user wasn't able to see any data.

We just spent an hour debugging a problem where a regular user was not seeing data for an index in Discover, but the elastic user could see it. We went through all the roles, role mappings, testing different permissions to try to bisect the problem, etc. What we eventually discovered was that we had inadvertently deleted the "*" in the Granted Fields section of the Role definition, so the user was able to see document metadata but none of the _source fields.

As soon as we went back and added the "" to Granted Fields it worked fine.
That box says it is "Optional", so shouldn't an empty box correlate to "", or at the very least throw a big red flag like "DO YOU REALLY WANT THIS BOX TO BE EMPTY BECAUSE YOU WON'T BE ABLE TO SEE ANY OF YOUR DATA". Maybe with the HTML blink tag or something.

[AlonaNadler]

cc: @elastic/kibana-security

[legrego]

Resolved via #19035