More null-semantics fixes.

The 'item' operator can now operate on an 'any' object. (Simulates dynamic typing, needed by Drill.)

Add "enabled" flag to SqlOperatorBaseTest (set to false by OptiqSqlOperatorTest) to mark out which tests (or parts of tests) currently pass.
Fix for JDK 1.6. (AutoCloseable does not exist until JDK 1.7.)