[BUGFIX] Check for valid BEuser record for avatar creation

A not valid BeUser record can not be retrieved from DB and must
not be passed to the AvatarViewHelper render() method to avoid
errors.

Resolves: #79104
Releases: master
Change-Id: I99ffedf9704f87b71e6ebeacc7136c31b0300fe4
Reviewed-on: https://review.typo3.org/51058


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>