[BUGFIX] Toolbar items with separator: fix white-space

The toolbar function could output erroneous html is case
additional toolbar items do not bring additionalAttributes.

Additional attributes are defined based on ToolbarItemHookInterface.
But additionalAttributes can be an empty string:

Fix output <liclass="separator"> to <li class="separator">
by adding an extra white-space.

Change-Id: I09338d17848c8fc74b25e7c532d6ee329956321b
Fixes: #45795
Releases: 6.0, 6.1
Reviewed-on: https://review.typo3.org/18458
Reviewed-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn