Optimize code to avoid undefined exception

Signed-off-by: gaobinlong <gbinlong@amazon.com>

Author: gaobinlong

opensearch_dashboards.json

@@ -31,3 +31,4 @@
     "opensearch-anomaly-detection"
   ]
 }
+

public/components/DiscoverAction/GenerateAnomalyDetector.tsx

@@ -99,7 +99,8 @@ function GenerateAnomalyDetector({
 
     const dataSourceId = context.indexPattern?.dataSourceRef?.id;
     const timeFieldFromIndexPattern = context.indexPattern?.timeFieldName;
-    const [categoricalFields, dateFields] = context.indexPattern?.fields.reduce(
+    const fieldsFromContext = context.indexPattern?.fields || [];
+    const [categoricalFields, dateFields] = fieldsFromContext.reduce(
         ([cFields, dFields], indexPatternField) => {
             const esType = indexPatternField.spec.esTypes?.[0];
             const name = indexPatternField.spec.name;