Alerting: Setting only key on custom label is not supported

### What happened? I tried to define additional key-only labels on an alert which will be sent to OpsGenie. ### What did you expect to happen? I expected to be able to define labels with only a key and no value. ### Did this work before? I am not sure. I personally never used this before. But I found this PR https://github.com/grafana/grafana/pull/30332 and the related issue https://github.com/grafana/grafana/issues/30331 where it seems possible to do what I want. ### How do we reproduce it? 1. Setup an Opsgenie contact point 2. Setup some alert 3. Try to add a label where you do not input anything in to the input field <img width="859" height="288" alt="Image" src="https://github.com/user-attachments/assets/5d848999-3d82-4c3b-aae7-d8c8dd6c154a" /> The UI does not allow to leave the value empty. Based on the linked PR/issue i thought it would be possible to define empty values however. ### Is the bug inside a dashboard panel? _No response_ ### Environment (with versions)? Grafana: 9.2.3 OS: Linux Browser: Firefox ### Grafana platform? None ### Datasource(s)? _No response_