[Bug][Investigations] - Fix timeline column reset (#188240)

## Summary

**Bug:**
Within the new timeline, we have default columns that show whenever an
empty array for columns are provided.
This was causing columns to re-appear in the `Selected fields` section
even though the columns were not actually selected anymore leading to
mismatched state between the browser ui and the table preventing the
default columns from being removed again without page refresh. _The old
timeline table is not affected_.



https://github.com/user-attachments/assets/7d16b8c3-be1a-4bc7-87b0-62fcac563f15



**Fix:**
The logic used to default an empty array of columns to the default
timeline columns has been updated to allow the empty array to be passed.
With the old timeline experience this just means all columns _can_ be
removed and you'd see an empty table, but with the new unified table
(which will be the default in 8.15 after


https://github.com/user-attachments/assets/e5fbaf48-77e3-4363-8232-35d256729b7e

x-pack/plugins/security_solution/public/common/mock/timeline_results.ts

@@ -2117,7 +2117,7 @@ export const defaultTimelineProps: CreateTimelineProps = {
     savedSearch: null,
     isDataProviderVisible: false,
     sampleSize: 500,
-    rowHeight: 1,
+    rowHeight: 3,
   },
   to: '2018-11-05T19:03:25.937Z',
   notes: null,

x-pack/plugins/security_solution/public/detections/components/alerts_table/actions.test.tsx

@@ -449,7 +449,7 @@ describe('alert actions', () => {
             savedSearchId: null,
             savedSearch: null,
             isDataProviderVisible: false,
-            rowHeight: 1,
+            rowHeight: 3,
             sampleSize: 500,
           },
           to: '2018-11-05T19:03:25.937Z',

x-pack/plugins/security_solution/public/timelines/components/timeline/body/unified_timeline_body.test.tsx

@@ -100,7 +100,7 @@ describe('UnifiedTimelineBody', () => {
 
     expect(MockUnifiedTimelineComponent).toHaveBeenLastCalledWith(
       expect.objectContaining({
-        columns: defaultUdtHeaders,
+        columns: [],
       }),
       {}
     );

x-pack/plugins/security_solution/public/timelines/components/timeline/body/unified_timeline_body.tsx

@@ -8,7 +8,6 @@
 import type { ComponentProps, ReactElement } from 'react';
 import React, { useEffect, useState, useMemo } from 'react';
 import { RootDragDropProvider } from '@kbn/dom-drag-drop';
-import { isEmpty } from 'lodash';
 import { StyledTableFlexGroup, StyledUnifiedTableFlexItem } from '../unified_components/styles';
 import { UnifiedTimeline } from '../unified_components';
 import { defaultUdtHeaders } from '../unified_components/default_headers';
@@ -59,9 +58,7 @@ export const UnifiedTimelineBody = (props: UnifiedTimelineBodyProps) => {
     });
   }, [events, pageInfo.activePage]);
 
-  const columnsHeader = useMemo(() => {
-    return isEmpty(columns) ? defaultUdtHeaders : columns;
-  }, [columns]);
+  const columnsHeader = useMemo(() => columns ?? defaultUdtHeaders, [columns]);
 
   return (
     <StyledTableFlexGroup direction="column" gutterSize="s">

x-pack/plugins/security_solution/public/timelines/components/timeline/tabs/shared/use_timeline_columns.test.ts

@@ -55,15 +55,15 @@ describe('useTimelineColumns', () => {
       const { result } = renderHook(() => useTimelineColumns([]), {
         wrapper: TestProviders,
       });
-      expect(result.current.localColumns).toEqual(defaultHeaders);
+      expect(result.current.localColumns).toEqual([]);
     });
 
     it('should return the default unified data table (udt) columns', () => {
       useIsExperimentalFeatureEnabledMock.mockReturnValue(false);
       const { result } = renderHook(() => useTimelineColumns([]), {
         wrapper: TestProviders,
       });
-      expect(result.current.localColumns).toEqual(defaultUdtHeaders);
+      expect(result.current.localColumns).toEqual([]);
     });
 
     it('should return the provided columns', () => {

x-pack/plugins/security_solution/public/timelines/components/timeline/tabs/shared/use_timeline_columns.tsx

@@ -5,7 +5,6 @@
  * 2.0.
  */
 
-import { isEmpty } from 'lodash/fp';
 import { useMemo } from 'react';
 import { SourcererScopeName } from '../../../../../sourcerer/store/model';
 import { useSourcererDataView } from '../../../../../sourcerer/containers';
@@ -28,10 +27,7 @@ export const useTimelineColumns = (columns: ColumnHeaderOptions[]) => {
     [unifiedComponentsInTimelineDisabled]
   );
 
-  const localColumns = useMemo(
-    () => (isEmpty(columns) ? defaultColumns : columns),
-    [columns, defaultColumns]
-  );
+  const localColumns = useMemo(() => columns ?? defaultColumns, [columns, defaultColumns]);
 
   const augmentedColumnHeaders = memoizedGetTimelineColumnHeaders(
     localColumns,

x-pack/plugins/security_solution/public/timelines/store/defaults.ts

@@ -102,7 +102,7 @@ export const timelineDefaults: SubsetTimelineModel &
   savedSearch: null,
   isDataProviderVisible: false,
   sampleSize: 500,
-  rowHeight: 1,
+  rowHeight: 3,
 };
 
 export const getTimelineManageDefaults = (id: string) => ({