NULL values are distinct. A comparison involving a NULL is always false.

NULL values are distinct.  A comparison involving a NULL is always false.
Operations on a NULL value yield a NULL result.  This change makes SQLite
operate more like the SQL spec, but it may break existing applications that
assumed the old behavior.  All the old tests pass but we still need to add
new tests to better verify the new behavior.  Fix for ticket #44. (CVS 589)

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