MDEV-35220 Assertion `!item->null_value' failed upon VEC_TOTEXT call

don't forget to reset null_value for each row

Author: vuvova

mysql-test/main/vector_funcs.result

@@ -74,3 +74,8 @@ select vec_fromtext(v) from t;
 drop table t;
 
 select vec_fromtext(0x00000000);
+
+--echo #
+--echo # MDEV-35220 Assertion `!item->null_value' failed upon VEC_TOTEXT call
+--echo #
+select vec_totext(`null`) from (values (null),(0x00000000)) x;

mysql-test/main/vector_funcs.test

@@ -65,11 +65,8 @@ bool Item_func_vec_totext::fix_length_and_dec(THD *thd)
 String *Item_func_vec_totext::val_str_ascii(String *str)
 {
   String *r1= args[0]->val_str();
-  if (args[0]->null_value)
-  {
-    null_value= true;
+  if ((null_value= args[0]->null_value))
     return nullptr;
-  }
 
   // Wrong size returns null
   if (r1->length() % 4)