[BUGFIX] add check for empty form values in FORM View

If a FORM is submitted without any value a Fatal Error is thrown in
typo3/sysext/form/Classes/View/Confirmation/Confirmation.php on line
101. The patch adds a check for that case.

Fixes: #28606
Releases: 6.1, 6.0, 4.7, 4.6

Change-Id: I77cd57f0d56e9bdcf9517683ea21c3dc9ec1bc45
Reviewed-on: https://review.typo3.org/11361
Reviewed-by: Alexander Stehlik
Tested-by: Alexander Stehlik
Reviewed-by: Stefan Neufeind
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn