[Fleet] Fix CSV download when no fields are chosen (#225368)

## Summary

Closes #218097 

- Added check to stop download button from being active when no fields
are selected as it doesnt make sense to download a CSV with zero fields.


![image](https://github.com/user-attachments/assets/0c69bade-a464-49a4-8276-1845e0f887e9)

### Checklist

Check the PR satisfies following conditions. 

Reviewers should verify this PR satisfies this list as well.

- [ ] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/src/platform/packages/shared/kbn-i18n/README.md)
- [ ]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [ ] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- [ ] If a plugin configuration key changed, check if it needs to be
allowlisted in the cloud and added to the [docker
list](https://github.com/elastic/kibana/blob/main/src/dev/build/tasks/os_packages/docker_generator/resources/base/bin/kibana-docker)
- [ ] This was checked for breaking HTTP API changes, and any breaking
changes have been approved by the breaking-change committee. The
`release_note:breaking` label should be applied in these situations.
- [ ] [Flaky Test
Runner](https://ci-stats.kibana.dev/trigger_flaky_test_runner/1) was
used on any tests changed
- [ ] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

### Identify risks

N/A

---------

Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>

x-pack/platform/plugins/shared/fleet/public/applications/fleet/sections/agents/components/agent_export_csv_modal/index.tsx

@@ -70,6 +70,7 @@ export const AgentExportCSVModal: React.FunctionComponent<Props> = ({
   return (
     <EuiConfirmModal
       data-test-subj="agentExportCSVModal"
+      confirmButtonDisabled={selection.length === 0}
       title={
         <FormattedMessage
           id="xpack.fleet.exportCSV.modalTitle"