[APM] Fix Cypress flaky test in Custom Links (#206108)

## Summary

Fixes #206016

This PR aims to fix a flaky test that waits for an empty state screen,
which sometimes fails to appear.

The issue might happen because the data deletion action, which triggers
the empty state, is not completed properly. This action takes place in
the previous test and does not wait for the deletion to finish.

The proposed solution ensures that the test responsible for deleting the
data waits until the empty state appears after deletion, preventing the
next test from running too soon.

### Test locally

`````
node x-pack/solutions/observability/plugins/apm/scripts/test/e2e.js --server
node x-pack/solutions/observability/plugins/apm/scripts/test/e2e.js  --runner --open
`````

Run the
[custom_links.cy.ts](http://localhost:5620/__/#/specs/runner?file=cypress/e2e/settings/custom_links.cy.ts)
test.

x-pack/solutions/observability/plugins/apm/ftr_e2e/cypress/e2e/settings/custom_links.cy.ts

@@ -109,14 +109,12 @@ describe('Custom links', () => {
       cy.visitKibana(basePath);
       cy.getByTestSubj('editCustomLink').click();
       cy.contains('Delete').click();
+      cy.getByTestSubj('customLinksEmptyPrompt').should('be.visible');
     });
 
     it('clears filter values when field is selected', () => {
       cy.visitKibana(basePath);
 
-      // wait for empty prompt
-      cy.getByTestSubj('customLinksEmptyPrompt').should('be.visible');
-
       cy.contains('Create custom link').click();
       cy.getByTestSubj('filter-0').select('service.name');
       cy.get('[data-test-subj="service.name.value"] [data-test-subj="comboBoxSearchInput"]').type(