mysql> create table t(a enum("a", "b", "c"));
Query OK, 0 rows affected (0.02 sec)

mysql> insert into t values('a'),('b');
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> set @@sql_mode='';
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> insert into t values("");
Query OK, 1 row affected, 1 warning (0.00 sec)

mysql> select * from t;
+------+
| a    |
+------+
| a    |
| b    |
|      |
+------+
3 rows in set (0.00 sec)

mysql> select * from t where a;
+------+
| a    |
+------+
| a    |
| b    |
+------+
2 rows in set (0.00 sec)