[v9.3.x] Alerting: Use optional chaining for accessing frames (#63417)

Co-authored-by: Gilles De Mey <gilles.de.mey@gmail.com>

public/app/features/alerting/unified/components/expressions/util.ts

@@ -1,11 +1,20 @@
 import { DataFrame, Labels, roundDecimals } from '@grafana/data';
 
+/**
+ * ⚠️ `frame.fields` could be an empty array ⚠️
+ *
+ * TypeScript will NOT complain about it when accessing items via index signatures.
+ * Make sure to check for empty array or use optional chaining!
+ *
+ * see https://github.com/Microsoft/TypeScript/issues/13778
+ */
+
 const getSeriesName = (frame: DataFrame): string => {
-  return frame.name ?? formatLabels(frame.fields[0].labels ?? {});
+  return frame.name ?? formatLabels(frame.fields[0]?.labels ?? {});
 };
 
 const getSeriesValue = (frame: DataFrame) => {
-  const value = frame.fields[0].values.get(0);
+  const value = frame.fields[0]?.values.get(0);
 
   if (Number.isFinite(value)) {
     return roundDecimals(value, 5);