to return NULL for non-NULL arguments.
This is not the case as it can return NULL
for invalid hexidecimal strings.
Fixed by setting the maybe_null flag.


mysql-test/r/func_str.result:
  Bug #26537: test case
mysql-test/t/func_str.test:
  Bug #26537: test case
sql/item_strfunc.h:
  Bug #26537: item_unhex() can return NULLs
  even for guaranteed non-null arguments.