Dashboard variables: Clear preview when re-ran

### What happened? When editing dashboard variables, the preview result persists between previews if the second preview ends up being an error. This can give the impression the second preview is valid and returns data, which it does not. ### What did you expect to happen? For the preview to reflect the results of the last click of the button, including if it was an error. ### Did this work before? N/A ### How do we reproduce it? Replication Steps: 1. Go here https://play.grafana.org/d/kr477t4/date-example 2. Click the preview button and see the data populate on the bottom. 3. Edit the `mysql_date` variable to remove the formatting. The query will now be `SELECT last_update FROM sakila.rental LIMIT 50` 4. Click preview again. Note the data from the previous preview still displays, even though this value is invalid (because it is a date) Note: if the play dashboard no longer exists, it is simply a dashboard with a query variable that uses a format function so the variable counts as a string and displays. The original query is `SELECT DATE_FORMAT(last_update, '%Y-%m-%d %H:%i:%s') FROM sakila.rental LIMIT 50` ### Is the bug inside a dashboard panel? No ### Environment (with versions)? Grafana: `main` ### Grafana platform? I use Grafana Cloud ### Datasource(s)? _No response_