Error if I do fetchall after 'CREATE TABLE' query

[HenryNebula]

Legacy issue: baztian/jaydebeapi#82

Tier: P0 — shared-bug

Calling fetchall/fetchone after a DDL statement (CREATE TABLE) that returns no ResultSet will still crash our codebase. convert_jdbc_rs_to_arrow_iterator receives a null/empty ResultSet and either JDBCUtils.convertResultSetToIterator or the subsequent iterator methods will fail. Our cursor implementation must guard against fetch calls when there is no result set.

[HenryNebula]

Triage fix: #77