[Alerting] Fix error when saving a rule after toggling alerts filter properties on and off (#192522)

## Summary

Closes #184170

Fixes a bug where the `alertsFilter` property gets added to `action`s
configured in the rule form when toggled on, but then never gets deleted
when toggled off. This would throw a validation error on the server for
an empty `alertsFilter` object, which is only meant to throw for
user-configured API requests and not for form usage.

### Checklist

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

## Release notes

Fixed a bug where toggling on and off the "If alert matches a query" or
"If alert is generated during timeframe" toggles makes it unable to save
the rule due to validation errors.

---------

Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>

x-pack/plugins/triggers_actions_ui/public/application/sections/rule_form/rule_reducer.test.ts

@@ -240,4 +240,38 @@ describe('rule reducer', () => {
     );
     expect(updatedRule.rule.alertDelay?.active).toBe(10);
   });
+
+  test('if rule action alerts filter was toggled on, then off', () => {
+    initialRule.actions.push({
+      id: '',
+      actionTypeId: 'testId',
+      group: 'Rule',
+      params: {},
+      uuid: '123-456',
+    });
+    let updatedRule = ruleReducer(
+      { rule: initialRule },
+      {
+        command: { type: 'setRuleActionAlertsFilter' },
+        payload: {
+          key: 'query',
+          value: 'hello',
+          index: 0,
+        },
+      }
+    );
+    expect((updatedRule.rule.actions[0] as SanitizedRuleAction).alertsFilter).toBeDefined();
+    updatedRule = ruleReducer(
+      { rule: initialRule },
+      {
+        command: { type: 'setRuleActionAlertsFilter' },
+        payload: {
+          key: 'query',
+          value: undefined,
+          index: 0,
+        },
+      }
+    );
+    expect((updatedRule.rule.actions[0] as SanitizedRuleAction).alertsFilter).toBeUndefined();
+  });
 });

x-pack/plugins/triggers_actions_ui/public/application/sections/rule_form/rule_reducer.ts

@@ -6,7 +6,7 @@
  */
 
 import { SavedObjectAttribute } from '@kbn/core/public';
-import { isEqual } from 'lodash';
+import { isEqual, isUndefined, omitBy } from 'lodash';
 import { Reducer } from 'react';
 import {
   RuleActionParam,
@@ -262,7 +262,7 @@ export const getRuleReducer =
             return state;
 
           const { alertsFilter, ...rest } = oldSanitizedAction;
-          const updatedAlertsFilter = { ...alertsFilter, [key]: value };
+          const updatedAlertsFilter = omitBy({ ...alertsFilter, [key]: value }, isUndefined);
 
           const updatedAction = {
             ...rest,