dblib either truncates nvarchar, or makes NULLs empty strings

Hello,
I am having two problems. If I set the tds version to 7.1, empty strings
are selected as NULL.

If I set the tds version to something else, strings longer than 255
characters are truncated.

My guess is that in the case of tds version 7.1, types marked with * at
http://www.freetds.org/tds.html#types are using "size == 0" when checking
for null, rather than the correct "size == -1". "size == 0" is an empty
string, whereas "size == -1" is NULL.

It isn't clear to me where in the source this check is happening, so I'm
applying here for help.

I've tried CentOS 7's FreeTDS .91 and also freetds-dev-0.92.965.

Thanks,
Jeff