[ES|QL] Set drop null columns param correctly for partial results (#1…

…92666)

## Summary

Closes #192595

We were not setting the `drop_null_columns ` queryString for partial
results and as a result it was returning the empty columns only for the
initial request, resulting in the weirdness that is being described in
the issue.

(cherry picked from commit 708a96c)

src/plugins/data/server/search/strategies/esql_async_search/esql_async_search_strategy.ts

@@ -79,7 +79,7 @@ export const esqlAsyncSearchStrategyProvider = (
             {
               method: 'GET',
               path: `/_query/async/${id}`,
-              querystring: { ...params },
+              querystring: { ...params, drop_null_columns: dropNullColumns },
             },
             { ...options.transport, signal: options.abortSignal, meta: true }
           )

test/functional/apps/discover/group6/_sidebar.ts

@@ -114,12 +114,17 @@ export default function ({ getService, getPageObjects }: FtrProviderContext) {
         options = await find.allByCssSelector('[data-test-subj*="typeFilter"]');
         expect(options).to.have.length(6);
 
-        expect(await unifiedFieldList.getSidebarAriaDescription()).to.be('82 available fields.');
+        expect(await unifiedFieldList.getSidebarAriaDescription()).to.be(
+          '76 available fields. 6 empty fields.'
+        );
 
         await testSubjects.click('typeFilter-number');
 
         await retry.waitFor('updates', async () => {
-          return (await unifiedFieldList.getSidebarAriaDescription()) === '6 available fields.';
+          return (
+            (await unifiedFieldList.getSidebarAriaDescription()) ===
+            '4 available fields. 2 empty fields.'
+          );
         });
       });
 
@@ -446,12 +451,16 @@ export default function ({ getService, getPageObjects }: FtrProviderContext) {
         await header.waitUntilLoadingHasFinished();
         await unifiedFieldList.waitUntilSidebarHasLoaded();
 
-        expect(await unifiedFieldList.getSidebarAriaDescription()).to.be('82 available fields.');
+        expect(await unifiedFieldList.getSidebarAriaDescription()).to.be(
+          '76 available fields. 6 empty fields.'
+        );
 
         await unifiedFieldList.clickFieldListItemRemove('extension');
         await unifiedFieldList.waitUntilSidebarHasLoaded();
 
-        expect(await unifiedFieldList.getSidebarAriaDescription()).to.be('82 available fields.');
+        expect(await unifiedFieldList.getSidebarAriaDescription()).to.be(
+          '76 available fields. 6 empty fields.'
+        );
 
         const testQuery = `from logstash-* | limit 10 | stats countB = count(bytes) by geo.dest | sort countB`;