[Discover] Fix flattening of _source fields of objects (elastic#136473)

- when fetching _source nested objects like obj.rootKey.subkey1, object.rootKey.subkey2 were not flattened correctly

(cherry picked from commit 0c78f8a)

# Conflicts:
#	x-pack/test/reporting_api_integration/reporting_and_security/__snapshots__/download_csv_dashboard.snap

src/plugins/data/common/search/tabify/tabify_docs.ts

@@ -94,7 +94,7 @@ export function flattenHit(hit: Hit, indexPattern?: IndexPattern, params?: Tabif
         continue;
       }
 
-      const hasValidMapping = field?.type !== 'conflict';
+      const hasValidMapping = field && field.type !== 'conflict';
       const isValue = !isPlainObject(val);
 
       if (hasValidMapping || isValue) {