Currently `exec_query` raises `NoMethodError` when executing no result
queries (`INSERT`, `UPDATE`, `DELETE`, and all DDL) in mysql2 adapter.

```
irb(main):002:0> conn.execute("create table t(a int)")
   (43.3ms)  create table t(a int)
=> nil
irb(main):003:0> conn.execute("insert into t values (1)")
   (19.3ms)  insert into t values (1)
=> nil
irb(main):004:0> conn.exec_query("insert into t values (1)")
  SQL (28.6ms)  insert into t values (1)
NoMethodError: undefined method `fields' for nil:NilClass
```