It seems I have already found the answer. We have now:
ERRD\_post\(Arg::Gds\(isc\_sqlerr\) << Arg::Num\(\-104\) <<
// Unexpected end of command
// CVC: Nothing will be line 1, column 1 for the user\.
Arg::Gds\(isc\_command\_end\_err2\) << Arg::Num\(1\) << Arg::Num\(1\)\);
i.e. "line 1, column 1" is reported when the user passes the NULL pointer. I believe it should be extended to handle the valid pointer but zero string length, so that the issue would be resolved automagically.