odbc driver compliance: NULL strings or "" in SQLTables and SQLColumns API calls

Stata fixed how they will talk to the MonetDB driver in the future, but they say the MonetDB driver is less standard, so perhaps we should be aware of this:

We were passing a NULL string to the ODBC

api call SQLTables and SQLColumns where the MonetDB driver wants an empty string (""). Some other drivers need a NULL string passed in to work (Oracle being one) so I believe this is a driver bug, but I have patched the problem in Stata. I have added code in -odbc- to check for the MonetDB driver and pass an empty string in that case.