[Visualizations] Add default filename when exporting CSV

[majagrubic]

….if visualization hasn't been saved

Summary

Fixes: #53960

When exporting a visualization, filename defaults to a visualization name. If visualization hasn't been saved previously, a file with an empty filename was exported, which led to confusing and erroneous behavior.

This PR introduces default (unsaved) filename, if visualization hasn't been saved previously.

Before:

After:

Tested in Firefox and Chrome.

Checklist

Use strikethroughs to remove checklist items you don't feel are applicable to this PR.

• This was checked for cross-browser compatibility, including a check against IE11
• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
  - [ ] Documentation was added for features that require explanation or tutorials
  - [ ] Unit or functional tests were updated or added to match the most common scenarios
• This was checked for keyboard-only and screenreader accessibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: c5d687a

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[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[myasonik]

Great little improvement!

[majagrubic]

thanks @myasonik 🙇‍♀