Sets the null key value for the specified index.
NINT ctDECL ctSetNullKeyValue(FILNO keyno, cpTEXT nullkey, ULONG keylen);
zero on success or a non-zero c-tree error code on failure.
COMPATIBILITY NOTE: An index file that uses this feature, and its corresponding data file, have a feature bit that is set in their header. If this feature bit is set and the server or client or standalone library attempting to open the file does not support this feature, the file open fails with error 744.