MariaRocks: rocksdb.rocksdb fails with a duplicate key error

In MariaDB, Field::make_sort_key stores NULL-indicator byte for
the field.
In MySQL, it doesn't, so MyRocks stores the NULL-indicator itself.

Switch to using Field::sort_string, which is the same as
Field::make_sort_key in MySQL.

storage/rocksdb/rdb_datadic.cc

@@ -741,7 +741,7 @@ void rdb_pack_with_make_sort_key(Rdb_field_packing *fpi, Field *field,
   DBUG_ASSERT(*dst != nullptr);
 
   const int max_len= fpi->m_max_image_len;
-  field->make_sort_key(*dst, max_len);
+  field->sort_string(*dst, max_len);
   *dst += max_len;
 }