Incorrect results when comparing `types.systemSetType` against other types

Dolt:
```sql
tmp/main> select @@sql_mode;
+---------------------------------------------------------------+
| @@sql_mode |
+---------------------------------------------------------------+
| NO_ENGINE_SUBSTITUTION,ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES |
+---------------------------------------------------------------+
1 row in set (0.00 sec)

tmp/main> select @@sql_mode = 0;
+----------------+
| @@sql_mode = 0 |
+----------------+
| false |
+----------------+
1 row in set (0.00 sec)

tmp/main> select @@sql_mode = 'asd';
value asd was not found in the set
```

MySQL:
```sql
mysql> select @@sql_mode;
+---------------------------------------------------------------+
| @@sql_mode |
+---------------------------------------------------------------+
| ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION |
+---------------------------------------------------------------+
1 row in set (0.0005 sec)

mysql> select @@sql_mode = 0;
+----------------+
| @@sql_mode = 0 |
+----------------+
| 1 |
+----------------+
1 row in set, 1 warning (0.0006 sec)
Warning (code 1292): Truncated incorrect DOUBLE value: 'ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION'

mysql> select @@sql_mode = 'asd';
+--------------------+
| @@sql_mode = 'asd' |
+--------------------+
| 0 |
+--------------------+
1 row in set (0.0015 sec)

```

Related: https://github.com/dolthub/go-mysql-server/pull/2842