[FixRM #8319] - Properly disable BLANK_PASSWORDS for ektron_cms400net

In module ektron_cms400net.rb, datastore option "BLANK_PASSWORDS" is
set to false by default, because according to the original author, a
blank password will result in account lockouts. Since the user should
never set "BLANK_PASSWORDS" to true, this option should never be
presented as an option (when issuing the "show options").

While fixing #8319, I also noticed another bug at line 108, where
res.code is used when res could be nil due to a timeout, so I ended
up fixing it, too.
Better fixes
Land #2259, @wchen-r7's patch for [SeeRM #8319]