wifi-scripts: don't fail on unset PSK

wifi-scripts: don't fail on unset PSK

Don't fail wireless interface bringup on empty PSK set. This is a valid
configuration, resulting in a PSK network which can't be connected to.
It does not fail the bringup of the hostapd process.

Keep failing the interface setup in case a password with invalid length
is used.

This is also beneficial when intending to configure a PPSK network. It
allows to create a network where no PPSK is yet set.

Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit 17a71f0c154cebd2e52cd8407531a9a88c931474)