[Unified Search] Disable "Load query" button when onClearSavedQuery…

… is undefined (#176756)

## Summary

This PR disables the "Load query" button in Unified Search when the
`onClearSavedQuery` callback is undefined. Previously it was possible to
click the "Load query" button, but an empty popover would be shown
instead of the saved query list, such as on the SLOs page.

SLOs page currently:


https://github.com/elastic/kibana/assets/25592674/3df21ac5-b727-4aa1-95c4-f81d7bf4a21b

"Load query" disabled:
<img width="914" alt="disabled"
src="https://github.com/elastic/kibana/assets/25592674/119dd5ea-1ee2-40c9-8b5c-09a1cee440cd">

This isn't really a fix to the underlying problem, it's just a quick
workaround to ensure the empty popover isn't shown. A full solution
would probably involve marking `onClearSavedQuery` as a required prop
and updating existing usages where its undefined, or updating Unified
Search to not display saved query actions if certain props aren't
defined.

### Checklist

- [ ] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/packages/kbn-i18n/README.md)
- [ ]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [ ] [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
- [ ] [Flaky Test
Runner](https://ci-stats.kibana.dev/trigger_flaky_test_runner/1) was
used on any tests changed
- [ ] Any UI touched in this PR is usable by keyboard only (learn more
about [keyboard accessibility](https://webaim.org/techniques/keyboard/))
- [ ] Any UI touched in this PR does not create any new axe failures
(run axe in browser:
[FF](https://addons.mozilla.org/en-US/firefox/addon/axe-devtools/),
[Chrome](https://chrome.google.com/webstore/detail/axe-web-accessibility-tes/lhdoppojpmngadmnindnejefpokejbdd?hl=en-US))
- [ ] If a plugin configuration key changed, check if it needs to be
allowlisted in the cloud and added to the [docker
list](https://github.com/elastic/kibana/blob/main/src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker)
- [ ] This renders correctly on smaller devices using a responsive
layout. (You can test this [in your
browser](https://www.browserstack.com/guide/responsive-testing-on-local-server))
- [ ] This was checked for [cross-browser
compatibility](https://www.elastic.co/support/matrix#matrix_browsers)

### For maintainers

- [ ] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

src/plugins/unified_search/public/query_string_input/query_bar_menu_panels.tsx

@@ -373,7 +373,7 @@ export function useQueryBarMenuPanels({
       panel: QueryBarMenuPanel.loadQuery,
       icon: 'filter',
       'data-test-subj': 'saved-query-management-load-button',
-      disabled: !hasSavedQueries,
+      disabled: !hasSavedQueries || !Boolean(manageFilterSetComponent),
     },
     {
       name: savedQuery ? strings.getSaveAsNewFilterSetLabel() : strings.getSaveFilterSetLabel(),

src/plugins/unified_search/public/search_bar/search_bar.tsx

@@ -543,7 +543,7 @@ class SearchBarUI<QT extends (Query | AggregateQuery) | Query = Query> extends C
         timeRangeForSuggestionsOverride={timeRangeForSuggestionsOverride}
         filtersForSuggestions={this.props.filtersForSuggestions}
         manageFilterSetComponent={
-          this.props.showFilterBar && this.state.query
+          this.props.showFilterBar && this.state.query && this.props.onClearSavedQuery
             ? this.renderSavedQueryManagement(
                 this.props.onClearSavedQuery,
                 this.props.showSaveQuery,
@@ -649,11 +649,11 @@ class SearchBarUI<QT extends (Query | AggregateQuery) | Query = Query> extends C
 
   private renderSavedQueryManagement = memoizeOne(
     (
-      onClearSavedQuery: SearchBarOwnProps['onClearSavedQuery'],
+      onClearSavedQuery: NonNullable<SearchBarOwnProps['onClearSavedQuery']>,
       showSaveQuery: SearchBarOwnProps['showSaveQuery'],
       savedQuery: SearchBarOwnProps['savedQuery']
     ) => {
-      const savedQueryManagement = onClearSavedQuery && (
+      const savedQueryManagement = (
         <SavedQueryManagementList
           showSaveQuery={showSaveQuery}
           loadedSavedQuery={savedQuery}