There is a convention that Item::val_int() and Item::val_real() return
SQL NULL doing effectively what this code does:
  null_value= true;
  return 0; // Always return 0 for SQL NULL

This is done to optimize boolean value evaluation:
if Item::val_int() or Item::val_real() returned 1 -
that always means TRUE and never can means SQL NULL.
This convention helps to avoid unnecessary testing
Item::null_value after getting a non-zero return value.

Item_func_min_max did not follow this convention.
It could return a non-zero value together with null_value==true.
This made evaluate_join_record() erroneously misinterpret
SQL NULL as TRUE in this call:

  select_cond_result= MY_TEST(select_cond->val_int());

Fixing Item_func_min_max to follow the convention.