removing unnecessary code in LengthValidator

this patch refactors LengthValidator code in activemodel

valid_value = if key == :maximum
 value.nil? || value.size.send(validity_check, check_value)
else
 value && value.size.send(validity_check, check_value)
end
 
next if valid_value

to much simpler

value ||= [] if key == :maximum
next if value && value.size.send(validity_check, check_value)