FS#2634 - Luci allows access if root password is cleared and locked

[openwrt-bot]

fblaese:

Luci doesn't correctly handle a disabled, empty password for root.
Deleting and disabling the root password makes the luci interface accessible without a password.

Steps to reproduce:

• Delete root password (passwd -d root)
• Lock root password (passwd -l root)
• Try to access Luci

[openwrt-bot]

jow-:

luci -> rpcd