Dashboards: Template variable queries failing result in variable references being blank

### What happened? This was observed with the Prometheus datasource, but might be happening elsewhere too. When the values for a template variable fail to be populated (e.g. the template variable queries prometheus for a list of label values, but that fails for some reason), the variable will be set to empty and the variable itself is set to a blank value. This means any queries that reference that variable will use that blank value. As an example, I created a Prometheus template variable with a fake function as the query: <img width="507" height="204" alt="Image" src="https://github.com/user-attachments/assets/e31b1dfe-25fd-4274-9de1-754fbe501c15" /> This meant the template variable query always fails: <img width="405" height="96" alt="Image" src="https://github.com/user-attachments/assets/7282ac77-f26d-4718-8b50-e6e2b65a2b9f" /> I had a panel with a Prometheus query referencing the template variable <img width="644" height="222" alt="Image" src="https://github.com/user-attachments/assets/e86bd794-fbff-4edc-87e1-81f2dc799f24" /> When inspecting the query that's sent to the Prometheus backend, the template variable value is blank: <img width="872" height="261" alt="Image" src="https://github.com/user-attachments/assets/f8565066-9955-4c3f-8e9a-e37534f6884e" /> I also noticed that when going to the dashboard (e.g. via the list of dashboards), the template variable is added to the url in the address bar with an empty value (e.g. `.../dashboard-with-failing-query` becomes `.../dashboard-with-failing-query?orgId=1&from=now-6h&to=now&timezone=utc&var-namespace=`). ### What did you expect to happen? I'm unsure if this is working as expected (and thus a feature request) or a bug, but my expectation that if a template variable query fails, any queries depending on the variable should not be executed, rather than being executed with a blank value. This can lead to unnecessary and possibly heavy queries to be executed. A blank value can be problematic in the Prometheus case, …