MDEV-14320 main.subselect failed in buildbot, results mismatch

There was a problem in the recent patch for MDEV-10817:
get_date() must "return true" on null_value rather than "return 0"

(forgot to fix Item_sum_hybrid::get_date() after copying and pasting
from Item_sum_hybrid::val_str()).

sql/item_sum.cc

@@ -2070,7 +2070,7 @@ Item_sum_hybrid::get_date(MYSQL_TIME *ltime, ulonglong fuzzydate)
 {
   DBUG_ASSERT(fixed == 1);
   if (null_value)
-    return 0;
+    return true;
   bool retval= value->get_date(ltime, fuzzydate);
   if ((null_value= value->null_value))
     DBUG_ASSERT(retval == true);