Skip ExecCheckRTPerms in CTAS with no data

Hi,

In case of CTAS with no data, we actually do not insert the tuples
into the created table, so we can skip checking for the insert
permissions. Anyways, the insert permissions will be checked when the
tuples are inserted into the table.

Attaching a small patch doing $subject.

Thoughts?

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com