fix(css): hide empty panel layout in detached mode

[dhayab]

Summary

In detached mode, typing a query that returns no results and for which the source does not include a noResults template shows an empty element with odd vertical space.

This is because shouldPanelOpen always returns true in detached mode, and while empty, the panel layout element has padding, which causes the issue.

This PR hides this element if it does not have children.

Before

After

Fixes #1191

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Sandbox	Source
@algolia/autocomplete-example-github-repositories-custom-plugin	Configuration
@algolia/autocomplete-example-instantsearch	Configuration
@algolia/autocomplete-example-playground	Configuration
@algolia/autocomplete-example-preview-panel-in-modal	Configuration
@algolia/autocomplete-example-react-renderer	Configuration
@algolia/autocomplete-example-starter-algolia	Configuration
@algolia/autocomplete-example-starter	Configuration
@algolia/autocomplete-example-reshape	Configuration
@algolia/autocomplete-example-vue	Configuration
dont-open-dropdown	Issue #1191