[ResponseOps][ES Query] Grouped over field is not populated correctly…

… when editing a rule (elastic#192297)

Resolves elastic#191077

## Summary

This PR fixes a bug loading the `groupby` field when editing an ES query
rule. While the form was loading fields from the index, the `fields`
array was empty ([]). This wasn't caught by a truthy check, so
`termField` was set to undefined as the fields array didn’t contain the
selected field. As a result, the form failed to populate correctly. To
fix this, I updated it to only check if the array contains the selected
field when the array is defined and also not empty.

I tested this for creating and editing the rule and it seemed to work,
pls let me know if there is another scenario I am missing with this fix.


### To verify

- Create an ES query rule with the `groupby` field set. See example
below of what the bug looked like before.
- Edit the rule and verify that the grouped by field is populated
correctly. I would test it a couple times to verify

**Example of Before:**
<img width="895" alt="Screen Shot 2024-09-06 at 2 09 43 PM"
src="https://github.com/user-attachments/assets/a03cebb5-4cc5-4d26-9e07-11e0ed120aef">

x-pack/plugins/triggers_actions_ui/public/common/expression_items/group_by_over.test.tsx

@@ -219,7 +219,7 @@ describe('group by expression', () => {
     expect(onChangeSelectedTermField).toHaveBeenCalledWith(['field1', 'field2']);
   });
 
-  it('do NOT clears selected agg field if fields is undefined', async () => {
+  it('do NOT clear selected agg field if fields is undefined', async () => {
     const onChangeSelectedTermField = jest.fn();
     render(
       <IntlProvider locale="en">
@@ -236,4 +236,22 @@ describe('group by expression', () => {
     );
     expect(onChangeSelectedTermField).toHaveBeenCalledTimes(0);
   });
+
+  it('do NOT clear selected agg field if fields is an empty array', async () => {
+    const onChangeSelectedTermField = jest.fn();
+    render(
+      <IntlProvider locale="en">
+        <GroupByExpression
+          errors={{ termSize: [], termField: [] }}
+          fields={[]}
+          termField={['test', 'unknown']}
+          groupBy={'top'}
+          onChangeSelectedGroupBy={() => {}}
+          onChangeSelectedTermSize={() => {}}
+          onChangeSelectedTermField={onChangeSelectedTermField}
+        />
+      </IntlProvider>
+    );
+    expect(onChangeSelectedTermField).toHaveBeenCalledTimes(0);
+  });
 });

x-pack/plugins/triggers_actions_ui/public/common/expression_items/group_by_over.tsx

@@ -109,7 +109,7 @@ export const GroupByExpression = ({
   }, [selectedTermsFieldsOptions, groupBy, onChangeSelectedTermField]);
 
   useEffect(() => {
-    if (fields) {
+    if (fields && fields.length > 0) {
       // if current field set doesn't contain selected field, clear selection
       const hasUnknownField = selectedTermsFieldsOptions.some(
         (fieldOption) => !fields.some((field) => field.name === fieldOption.label)