[BUGFIX] Invalid HTML for the top toolbar in Backend

Additional attributes must be concatenated with a leading blank
space in order to get valid HTML content.

Fixes: #57127
Releases: 6.2
Change-Id: Iac0cf3033af37efd8e1a8efeb777083503166c5c
Reviewed-on: https://review.typo3.org/28587
Reviewed-by: Andreas Fernandez
Tested-by: Andreas Fernandez
Reviewed-by: Eric Chavaillaz
Tested-by: Eric Chavaillaz
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers