[Discover] Fix resetting of breakdown field in a saved search (elasti…

…c#184668)

- Closes elastic#184379

## Summary

This PR brings back the logic which was edited in
elastic#169548 This should allow to reset
the saved breakdown field with rather an empty string as `undefined`
seems to be ignored.

### 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

src/plugins/discover/public/application/main/state_management/discover_saved_search_container.ts

@@ -408,6 +408,10 @@ function getSavedSearchFieldForComparison(
     return visContext;
   }
 
+  if (fieldName === 'breakdownField') {
+    return savedSearch.breakdownField || ''; // ignore the difference between an empty string and undefined
+  }
+
   return savedSearch[fieldName];
 }
 

src/plugins/discover/public/application/main/state_management/discover_state.test.ts

@@ -453,7 +453,6 @@ describe('Test discover state actions', () => {
     const { searchSource, ...savedSearch } = state.savedSearchState.getState();
     expect(savedSearch).toMatchInlineSnapshot(`
       Object {
-        "breakdownField": undefined,
         "columns": Array [
           "default_column",
         ],

src/plugins/discover/public/application/main/state_management/utils/update_saved_search.test.ts

@@ -124,6 +124,40 @@ describe('updateSavedSearch', () => {
     });
   });
 
+  it('should pass breakdownField if state has breakdownField', async () => {
+    const savedSearch = {
+      ...savedSearchMock,
+      searchSource: savedSearchMock.searchSource.createCopy(),
+    };
+    expect(savedSearch.breakdownField).toBeUndefined();
+    updateSavedSearch({
+      savedSearch,
+      globalStateContainer: createGlobalStateContainer(),
+      services: discoverServiceMock,
+      state: {
+        breakdownField: 'test',
+      },
+    });
+    expect(savedSearch.breakdownField).toEqual('test');
+  });
+
+  it('should pass an empty string if state already has breakdownField', async () => {
+    const savedSearch = {
+      ...savedSearchMock,
+      searchSource: savedSearchMock.searchSource.createCopy(),
+      breakdownField: 'test',
+    };
+    updateSavedSearch({
+      savedSearch,
+      globalStateContainer: createGlobalStateContainer(),
+      services: discoverServiceMock,
+      state: {
+        breakdownField: undefined,
+      },
+    });
+    expect(savedSearch.breakdownField).toEqual('');
+  });
+
   it('should set query and filters from services', async () => {
     const savedSearch = {
       ...savedSearchMock,

src/plugins/discover/public/application/main/state_management/utils/update_saved_search.ts

@@ -74,7 +74,12 @@ export function updateSavedSearch({
       savedSearch.viewMode = state.viewMode;
     }
 
-    savedSearch.breakdownField = state.breakdownField || undefined; // `undefined` instead of an empty string
+    if (typeof state.breakdownField !== 'undefined') {
+      savedSearch.breakdownField = state.breakdownField;
+    } else if (savedSearch.breakdownField) {
+      savedSearch.breakdownField = '';
+    }
+
     savedSearch.hideAggregatedPreview = state.hideAggregatedPreview;
 
     // add a flag here to identify ES|QL queries