new password shouldn't be empty string

enter "Home screen backup password" in "Developer Settings",
do nothings and then click "Set backup password", There will
be a toast display "New backup password set".
The new password field cannot be empty.

Bug: https://partnerissuetracker.corp.google.com/u/2/issues/111261338


Test: empty string can not be set as a password

Signed-off-by: xiamin <xiamin@xiaomi.com>