TAJO-948: 'INSERT INTO' statement to non existence table casuses NPE

The cause is there is no part to check if the table instance is null before it's used.

Please review if the changeset is appropriate.

Thanks.
The bug fix looks correct. But, I have one suggestion about the message. The other similar cases use ```relation "%s" does not exist```. Could you change the message for consistency?

You can see the similar case at https://github.com/eminency/tajo/blob/TAJO-948/tajo-core/src/main/java/org/apache/tajo/engine/planner/PreLogicalPlanVerifier.java#L134