[BUGFIX] Empty clear cache command logged

If e.g. enabling/disabling a record in the backend, an
additional empty clear cache command is logged, like
"User admin has cleared the cache (cacheCmd=)".

Change-Id: Ie7b6ee47c9e53c7ba399aada1f738fe700574e89
Fixes: #47181
Releases: 6.0, 6.1
Reviewed-on: https://review.typo3.org/19902
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn