[FixRM #8525] undefined method `+' for nil:NilClass in enum_ie

Looks like for some reason if CryptUnprotectData fails, the decrypt_reg()
method will return "". And when you unpack "", you produce an array of nils.
Since you cannot add something to nil, this should cause an
"undefined method `+' for nil:NilClass" error.

This will check if we get an array of nils, we jump to the next iteration.
Check dec instead
Land #2578, @wchen-r7's [FixRM #8525]