server: Correct the check for Resultset

[dveeden]

This is likely a regression introduced by #969

Found by running this code: #915 (comment)

Queries like SELECT 1 were working correctly, but DO 1 and SET ... were not as the code tried to write a resultset instead of an OK.

[dveeden]

Without this PR:

mysql-8.0.11> SELECT 1;
+---+
| 1 |
+---+
| 1 |
+---+
1 row in set (0.00 sec)

mysql-8.0.11> DO 1;
ERROR 2027 (HY000): Malformed packet

With this PR:

mysql-8.0.11> SELECT 1;
+---+
| 1 |
+---+
| 1 |
+---+
1 row in set (0.00 sec)

mysql-8.0.11> DO 1;
Query OK, 0 rows affected (0.00 sec)