Heatmap: Fix runtime error when no data is present, add e2e test (#113242)

* Heatmap: Fix runtime error when no xAxis is present, add e2e test

* update migration test fixture

* updates from review

apps/dashboard/pkg/migration/testdata/dev-dashboards-output/panel-heatmap/heatmap-x.v42.json

@@ -303,6 +303,70 @@
       ],
       "title": "Cells heatmap",
       "type": "heatmap"
+    },
+    {
+      "datasource": {
+        "type": "datasource",
+        "uid": "-- Dashboard --"
+      },
+      "fieldConfig": {
+        "defaults": {
+          "custom": {
+            "hideFrom": {
+              "legend": false,
+              "tooltip": false,
+              "viz": false
+            },
+            "scaleDistribution": {
+              "type": "linear"
+            }
+          }
+        },
+        "overrides": []
+      },
+      "gridPos": {
+        "h": 8,
+        "w": 12,
+        "x": 0,
+        "y": 17
+      },
+      "id": 7,
+      "options": {
+        "calculate": false,
+        "cellGap": 1,
+        "color": {
+          "exponent": 0.5,
+          "fill": "dark-orange",
+          "mode": "scheme",
+          "reverse": false,
+          "scale": "exponential",
+          "scheme": "Oranges",
+          "steps": 64
+        },
+        "exemplars": {
+          "color": "rgba(255,0,255,0.7)"
+        },
+        "filterValues": {
+          "le": 1e-9
+        },
+        "legend": {
+          "show": true
+        },
+        "rowsFrame": {
+          "layout": "auto"
+        },
+        "tooltip": {
+          "show": true,
+          "yHistogram": false
+        },
+        "yAxis": {
+          "axisPlacement": "left",
+          "reverse": false
+        }
+      },
+      "pluginVersion": "12.3.0-pre",
+      "title": "No data",
+      "type": "heatmap"
     }
   ],
   "refresh": "",
@@ -320,4 +384,4 @@
   "title": "Heatmap X axis",
   "uid": "5Y0jv6pVz",
   "weekStart": ""
-}
+}