Fixup ODBC :connection-string codepath.

In the call to odbc's SQLDriverConnect default the WindowHandle
argument to a null ptr so that connecting with :connection-string will
work in the default case of SQL_DRIVER_NOPROMPT.

I.e. you can now do things like: (clsql:connect '("DsnName" "UserName"
"" :connection-string
"DRIVER={FreeTDS};SERVER=...;DATABASE=...;UID=...;PWD=...;PORT=1433;TDS_Version=8.0;APP=clsql"
:database-type :odbc))

I believe the DsnName and Username at that point are only used when
printing the connection information.

When connecting with ODBC :connection-string argument default the
window-handle (for popping up a prompt) to the null ptr in the case
that one hasn't been passed in.

The default is to tell the driver library not to prompt so it doesn't
really need a window handle, it just needs it to be a null ptr rather
than nil.