Allow to pass empty email to admin command line

0cffbe50bab4 added the validation of the email in trytond-admin but it removed the possibility to use empty value. See https://discuss.tryton.org/t/i-hate-docker-but-i-did-give-it-a-try-and-no-database/8240

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information