MDEV-28384 UBSAN: null pointer passed as argument 1, which is declared to never be null in my_strnncoll_binary on SELECT ... COUNT or GROUP_CONCAT

Also fixes:
  MDEV-30982 UBSAN: runtime error: null pointer passed as argument 2, which is declared to never be null in my_strnncoll_binary on DELETE

Calling memcmp() with a NULL pointer is undefined behaviour
according to the C standard, even if the length argument is 0.

Adding tests for length==0 before calling memcmp() into:
- my_strnncoll_binary()
- my_strnncoll_8bit_bin