Graph (old): fix null interval insertion, again (follow-up to #46069) (#46139) (#46141)

(cherry picked from commit 89140c8353)

Co-authored-by: Leon Sorokin <leeoniya@gmail.com>

public/app/plugins/panel/graph/data_processor.ts

@@ -32,13 +32,14 @@ export class DataProcessor {
 
     for (let i = 0; i < dataList.length; i++) {
       let series = dataList[i];
-      const { timeField } = getTimeField(series);
+      let { timeField } = getTimeField(series);
 
       if (!timeField) {
         continue;
       }
 
       series = applyNullInsertThreshold(series, timeField.name);
+      timeField = getTimeField(series).timeField!; // use updated length
 
       for (let j = 0; j < series.fields.length; j++) {
         const field = series.fields[j];