MDEV-24975 Server consumes extra 4G memory upon querying INFORMATION_SCHEMA.OPTIIMIZER_TRACE

if a query used no fields from an I_S table, we were creating a temp
table with one, first, field (as a table cannot have zero fields),
with its length truncated to 1.

Now - force also this dummy field to be a normal field, not a BLOB

sql/sql_show.cc

@@ -8767,6 +8767,7 @@ bool optimize_schema_tables_memory_usage(List<TABLE_LIST> &tables)
       {
         /* all fields were optimized away. Force a non-0-length row */
         table->s->reclength= to_recinfo->length= 1;
+        to_recinfo->type= FIELD_NORMAL;
         to_recinfo++;
       }
       p->recinfo= to_recinfo;