Added test for sorting zero-sized vectors:

- CMath::qsort(* T, ...)
- CMath::qsort(** T, ...)
- CMath::qsort_index(...)
- CMath::qsort_backward_index(...)
- CMath::parallel_qsort_index(...)

Prevented potential segfaults when sorting zero-sized vectors:

- CMath::qsort(** T)
- CMath::qsort_index(...)
- CMath::qsort_backward_index(...)
- CMath::parallel_qsort_index(...)