[Advanced Settings] Fix defaultIndex setting's default value (#170865)

Fixes #168732

## Summary

This PR changes the default value of the `defaultIndex` setting to an
empty string. Previously, the default value was set to `null` and this
was causing a wrong behaviour in the Advanced Settings UI as it expects
a string value for the settings of type `string`.

**Before - the "Reset to default" link did not clear the user value:**


https://github.com/elastic/kibana/assets/59341489/2c47306b-feac-4e08-bbae-3db7b0a3e5fd


**With these changes:**


https://github.com/elastic/kibana/assets/59341489/dc46caa8-9860-4b66-87ca-9c184c5a8391

src/plugins/data/server/ui_settings.ts

@@ -168,12 +168,12 @@ export function getUiSettings(
       name: i18n.translate('data.advancedSettings.defaultIndexTitle', {
         defaultMessage: 'Default data view',
       }),
-      value: null,
+      value: '',
       type: 'string',
       description: i18n.translate('data.advancedSettings.defaultIndexText', {
         defaultMessage: 'Used by discover and visualizations when a data view is not set.',
       }),
-      schema: schema.nullable(schema.string()),
+      schema: schema.string(),
     },
     [UI_SETTINGS.COURIER_IGNORE_FILTER_IF_FIELD_NOT_IN_INDEX]: {
       name: i18n.translate('data.advancedSettings.courier.ignoreFilterTitle', {

test/api_integration/apis/data_views/default_index_pattern/default_index_pattern.ts

@@ -48,7 +48,7 @@ export default function ({ getService }: FtrProviderContext) {
           expect(response5.status).to.be(200);
 
           const response6 = await supertest.get(defaultPath);
-          expect(response6.body[serviceKeyId]).to.be(null);
+          expect(response6.body[serviceKeyId]).to.be('');
         });
       });
     });

x-pack/test_serverless/api_integration/test_suites/common/data_views/default_index_pattern/default_index_pattern.ts

@@ -72,7 +72,7 @@ export default function ({ getService }: FtrProviderContext) {
           // TODO: The response comes back undefined in Serverless
           const body = response6.body[serviceKeyId];
           const expected = body === undefined ? null : body;
-          expect(expected).to.be(null);
+          expect(expected).to.be('');
         });
       });
     });