[BUGFIX] EXT:form Validation of email

Validation of email should respect empty value

Resolves: #54534
Releases: master,7.6
Change-Id: I40c865b42fe7960dd7ab85f710df1045cd04adf0
Reviewed-on: https://review.typo3.org/47059


Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Jan Helke <typo3@helke.de>
Tested-by: Jan Helke <typo3@helke.de>