Description of problem:
I found what seems a corner case where lightdm allows the login even if the provided password is actually "wrong". This might not be a true bug, but *in my opinion* the behavior of lightdm is not what an user expects.

Version-Release number of selected component (if applicable):
lightdm-1.10.5-4.fc23.i686

How reproducible:
Always

Steps to Reproduce:
1. Create an user without password.
2. Login through lightdm with that user.
3. Provide a password for the login.

Actual results:
ligthdm allows the user to login.

Expected results:
At least in my opinion, lightdm should refuse the login. The real password is blank, so if no blank password is provided, the real combination of "login name + password" is not actually matched.

Additional info:
Fedora 23 Final TC1 (32 bit) on qemu-kvm.
lxdm has got the same approach (See bug 1268624).