[BUGFIX] Prevent PHP warning on entering an empty install tool password

Show a useful message if an empty install tool password is entered.

Resolves: #81936
Releases: master, 8.7
Change-Id: I17aaf2aaeb5b31a68c8701cdaa560c00364bea1e
Reviewed-on: https://review.typo3.org/53613


Tested-by: TYPO3com <no-reply@typo3.com>
Tested-by: Renzo Bauen <info@conpassione.ch>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Sascha Egerer <sascha@sascha-egerer.de>
Tested-by: Sascha Egerer <sascha@sascha-egerer.de>
Reviewed-by: Sascha Rademacher <sascha.rademacher+typo3@gmail.com>
Tested-by: Sascha Rademacher <sascha.rademacher+typo3@gmail.com>
Reviewed-by: Henning Liebe <h.liebe@neusta.de>
Tested-by: Henning Liebe <h.liebe@neusta.de>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Tested-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>