Function `SetSubtaskInfo` takes only chars as a valid book. Since
filename can be `nil` (not a char) the crash would happen. This
Change avoids the crash by using a char type nil value. See:
`int SetSubtaskInfo(int task, int subtask, const char *name, const char *book);`
Fixes#8053Fixes#8392 too
Fixes#8743 last issue