Dashboards: heatmap displays 'TypeError: can't access property "config", f is undefined' when query returns no data

### What happened? Under Grafana 12.1.0, heatmap visualizations using a query which returns no data (E.g., nothing was observed in the range) display an error message that suggests a configuration problem when this is an ordinary state of affairs. <img width="879" height="345" alt="Image" src="https://github.com/user-attachments/assets/9eaa197a-6fd0-4479-9c31-d5bf86c6d9a0" /> ### What did you expect to happen? A blank display similar to how a timeseries visualization behaves when there is no data in the range. ### Did this work before? Unknown. This has only been tested with 12.1.0. ### How do we reproduce it? 1.create a heat map using a prometheus native histogram metric 2.change the range to a period in which there are no observations. ### Is the bug inside a dashboard panel? | Key | Value | |--|--| | Panel | heatmap @ 12.1.0 | | Grafana | Grafana v12.1.0 (ccd7b6ce7e) // Open Source | <details><summary>Panel debug snapshot dashboard</summary> ```json { "panels": [ { "id": 2, "type": "heatmap", "title": "Reproduced with embedded data", "description": "The total duration of the request including any time queued waiting for upload slot(s).", "gridPos": { "h": 13, "w": 15, "x": 0, "y": 0 }, "fieldConfig": { "defaults": { "custom": { "scaleDistribution": { "type": "linear" }, "hideFrom": { "tooltip": false, "viz": false, "legend": false } } }, "overrides": [] }, "pluginVersion": "12.1.0", "targets": [ { "refId": "A", "datasource": { "type": "grafana", "uid": "grafana" }, "queryType": "snapshot", "snapshot": [ { "schema": { "refId": "B", "meta": { "typeVersion": [ …